Nucleus and nuclear transport: nuclear pore complex, importins, exportins, and chromatin organization

Intuition Beginner

The nucleus is the command center of a eukaryotic cell. A double membrane called the nuclear envelope separates your DNA from the rest of the cell. But this barrier is not sealed shut — it is studded with thousands of nuclear pore complexes (NPCs) that act as controlled gateways between the nucleus and the cytoplasm.

Small molecules like water and ions pass through the NPC freely. But large proteins and RNA need special permission. Proteins destined for the nucleus carry a nuclear localization signal (NLS) — a short amino acid sequence that acts like a zip code. Transport proteins called importins recognize this signal and guide the cargo through the pore. In the opposite direction, exportins carry RNA and proteins out of the nucleus.

Inside the nucleus, DNA wraps around histone proteins to form chromatin. Loosely packed chromatin (euchromatin) contains actively transcribed genes, while tightly packed chromatin (heterochromatin) keeps genes silent. This packaging system determines which genes a cell can read at any given time.

Visual Beginner

The nuclear pore complex is a massive assembly of roughly 1,000 protein molecules arranged with eight-fold rotational symmetry. FG-repeat nucleoporins (shown as flexible filaments extending into the channel) form a selective sieve. Small molecules diffuse through freely, while cargo proteins bearing a nuclear localization signal are ferried through by importins. The Ran GTPase provides directionality: Ran-GTP is concentrated in the nucleus (gold) and Ran-GDP in the cytoplasm (blue).

Worked example Beginner

Tracing the import of a transcription factor into the nucleus.

Consider a transcription factor called NF-kB that needs to enter the nucleus to activate immune response genes. In its inactive state, NF-kB is held in the cytoplasm by an inhibitor protein called IkB, which masks the NLS on NF-kB.

Step 1. Signal reception. When a cell detects a bacterial molecule (such as LPS), a signaling cascade activates the kinase IKK, which phosphorylates IkB.

Step 2. Inhibitor degradation. Phosphorylated IkB is recognized by a ubiquitin ligase, tagged with ubiquitin, and destroyed by the proteasome. This exposes the NLS on NF-kB.

Step 3. Importin binding. The exposed NLS is recognized by importin-alpha. Importin-alpha then binds importin-beta, forming a trimeric import complex.

Step 4. Transit through the NPC. The import complex interacts with FG-repeat nucleoporins inside the pore channel and translocates through the NPC into the nucleus.

Step 5. Cargo release. Inside the nucleus, Ran-GTP binds to importin-beta, causing a conformational change that releases both importin-alpha and NF-kB. The transcription factor is now free to bind DNA and activate gene expression.

Step 6. Receptor recycling. Importin-beta bound to Ran-GTP is exported back to the cytoplasm. There, a GTPase-activating protein (RanGAP) converts Ran-GTP to Ran-GDP, releasing importin-beta for another round of import.

Check your understanding Beginner

Formal definition Intermediate+

Nuclear pore complex (NPC). The NPC is an approximately 120 MDa protein assembly embedded in the nuclear envelope at sites where the inner and outer nuclear membranes are fused. Each NPC contains multiple copies of roughly 30 distinct proteins called nucleoporins (Nups), arranged with eight-fold rotational symmetry and two-fold symmetry across the nuclear envelope plane. The central scaffold forms a ring approximately 120 nm in diameter with a central channel roughly 40-50 nm wide at rest. The cytoplasmic face has eight cytoplasmic filaments, and the nuclear face has a basket-like structure (the nuclear basket) formed by Nup153 and Tpr.

FG-repeat nucleoporins. Approximately one-third of nucleoporins contain repeats of the dipeptide phenylalanine-glycine (FG). These FG-Nups line the central channel and extend flexible filaments into the lumen, forming a disordered hydrophobic mesh that functions as a selective diffusion barrier. The FG repeats bind directly to transport receptors (karyopherins), allowing cargo-receptor complexes to dissolve through the mesh while excluding inert macromolecules above roughly 40-60 kDa.

Nuclear localization signal (NLS) and nuclear export signal (NES). The classical NLS is a stretch of basic residues, classified as either monopartite (a single cluster, e.g., PKKKRKV from SV40 large T antigen) or bipartite (two clusters separated by a 10-12 residue spacer, e.g., KRPAATKKAGQAKKK from nucleoplasmin). The classical NES is a leucine-rich sequence conforming to the consensus L-x(2,3)-[LIVFM]-x(2,3)-L-x(1)-[LIVFM] (where x is any amino acid), recognized by the exportin CRM1 (chromosome region maintenance 1, also called Exportin-1/Xpo1).

Karyopherins (importins and exportins). Karyopherins are a family of roughly 20 transport receptors in humans that shuttle cargo through the NPC. Importins bind cargo in the cytoplasm and release it in the nucleus. The classical import pathway uses a heterodimer: importin-alpha (adaptor that binds the NLS) and importin-beta (the transport receptor that interacts with FG-Nups). Importin-beta can also bind cargoes directly via non-classical NLS motifs. Exportins bind cargo in the nucleus (in the presence of Ran-GTP) and release it in the cytoplasm upon GTP hydrolysis. CRM1 is the major exportin for leucine-rich NES-bearing proteins; exportin-t (Xpo-t) exports tRNAs; and exportin-5 exports pre-miRNAs.

Ran GTPase cycle. Ran (Ras-related nuclear protein) is a small GTPase that exists in two states: Ran-GTP (active) and Ran-GDP (inactive). The asymmetric distribution of Ran's regulators creates a Ran-GTP gradient:

Component	Location	Function
RanGEF (RCC1)	Nucleus (bound to chromatin)	Exchanges GDP for GTP on Ran
RanGAP + RanBP1/2	Cytoplasm	Stimulates GTP hydrolysis on Ran
NTF2	Cytoplasm to nucleus	Imports Ran-GDP back into the nucleus

Ran-GTP is concentrated in the nucleus because RCC1 continuously recharges Ran-GDP. Ran-GDP predominates in the cytoplasm because RanGAP drives hydrolysis. This gradient provides the thermodynamic driving force for directional transport.

Chromatin organization. Chromatin exists in two major functional states. Euchromatin is decondensed, enriched in active histone marks (H3K4me3, H3K27ac), and transcriptionally permissive. Heterochromatin is condensed, enriched in repressive marks (H3K9me3, H3K27me3), and transcriptionally silent. Heterochromatin is subdivided into constitutive heterochromatin (permanently silent regions such as centromeres and telomeres, marked by H3K9me3 and HP1 binding) and facultative heterochromatin (developmentally regulated silencing, such as the inactive X chromosome, marked by H3K27me3 and Polycomb binding). Chromatin is further organized into chromosome territories — discrete, non-random spatial positions occupied by each chromosome within the nucleus.

Nuclear lamina. The nuclear lamina is a dense fibrillar network (approximately 30-40 nm thick) of intermediate filament proteins (lamins) lining the inner surface of the nuclear envelope. In mammals, A-type lamins (lamin A and lamin C, splice variants of LMNA) and B-type lamins (lamin B1 from LMNB1, lamin B2 from LMNB2) polymerize into a meshwork that provides mechanical support to the nucleus and serves as an anchoring platform for chromatin.

Key mechanism Intermediate+

The Ran-GTP gradient drives directional nuclear transport

The fundamental problem of nuclear transport is directionality: importins must pick up cargo in the cytoplasm and release it in the nucleus, while exportins do the reverse. The Ran GTPase provides the energy and asymmetry for both processes.

Import cycle (importin-alpha/beta pathway).

1. Cargo binding (cytoplasm). Importin-alpha recognizes the NLS on a cargo protein. Importin-beta binds importin-alpha, forming a trimeric complex. In the cytoplasm, Ran-GDP levels are high and Ran-GTP levels are low, so importin-beta is free to bind importin-alpha.

2. Transit through the NPC. The import complex binds FG-repeat nucleoporins within the NPC channel. Importin-beta has hydrophobic pockets that interact with FG repeats, allowing the complex to dissolve through the mesh by a series of binding and release events. This process does not consume ATP or GTP during translocation — the energy comes from the binding energy between importin-beta and FG repeats.

3. Cargo release (nucleus). Inside the nucleus, Ran-GTP binds importin-beta at a site that overlaps with the importin-alpha binding interface. This causes a conformational change: importin-beta releases importin-alpha. Importin-alpha is then bound by CAS (cellular apoptosis susceptibility protein, also called Cse1p in yeast), an exportin that returns importin-alpha to the cytoplasm in complex with Ran-GTP.

4. Receptor recycling (cytoplasm). The importin-beta/Ran-GTP complex diffuses back through the NPC to the cytoplasm. There, RanGAP (stimulated by RanBP1 and RanBP2/Nup358 at the cytoplasmic face of the NPC) triggers GTP hydrolysis. Ran-GDP dissociates from importin-beta, freeing the receptor for another import cycle. NTF2 mediates the return of Ran-GDP to the nucleus, where RCC1 recharges it to Ran-GTP.

Export cycle (CRM1 pathway).

1. Cargo binding (nucleus). CRM1 recognizes an NES-bearing cargo. Unlike importins, CRM1 only binds its cargo with high affinity in the presence of Ran-GTP. A trimeric export complex forms: CRM1 + cargo + Ran-GTP.

2. Transit through the NPC. The trimeric complex interacts with FG-Nups and translocates to the cytoplasm.

3. Cargo release (cytoplasm). RanGAP and RanBP1 at the cytoplasmic face trigger GTP hydrolysis on Ran. The resulting Ran-GDP has low affinity for CRM1, causing the export complex to disassemble. The cargo is released into the cytoplasm.

4. Receptor recycling. CRM1 returns to the nucleus (its low affinity for FG-Nups in the absence of Ran-GTP allows passive diffusion back through the NPC). Ran-GDP is imported by NTF2 for recharging by RCC1.

The net directionality arises because RCC1 is exclusively nuclear and RanGAP is exclusively cytoplasmic. Each complete import or export cycle consumes one molecule of GTP (hydrolyzed when Ran-GTP reaches the cytoplasm). At approximately 1,000 NPCs per mammalian cell and an estimated transport rate of roughly 1,000 transport events per second per NPC, nuclear transport is one of the most energetically demanding processes in the cell.

Chromatin compartmentalisation

The spatial organization of chromatin within the nucleus is non-random. Key organizing principles include:

• Chromosome territories: Each chromosome occupies a distinct, roughly spherical domain. Gene-rich chromosomes tend to be positioned toward the nuclear interior, while gene-poor chromosomes cluster at the periphery.
• Lamina-associated domains (LADs): Large chromatin regions (0.1-10 Mb) that physically contact the nuclear lamina. LADs are generally gene-poor, enriched in heterochromatin marks (H3K9me2/3), and transcriptionally silent. Approximately 30-40% of the genome is organized into LADs.
• Nucleolar organizer regions (NORs): Chromosomal regions containing ribosomal DNA repeats that cluster around the nucleolus, the site of ribosomal RNA synthesis and ribosome subunit assembly.
• Transcription factories: Clusters of active RNA polymerase II molecules that concentrate at focal points in the nucleus. Genes from different chromosomes can co-localize at shared transcription factories when co-regulated.

Exercises Intermediate+

Nuclear pore complex architecture and transport thermodynamics Master

Cryo-electron microscopy and cryo-electron tomography have resolved the NPC structure at sub-nanometer resolution, revealing an architecture of extraordinary complexity. The human NPC is built from roughly 1,000 individual protein molecules comprising approximately 30 distinct nucleoporins (Nups), each present in 8, 16, or 32 copies per NPC. The total mass is approximately 110-125 MDa in vertebrates, making it one of the largest protein assemblies in the cell. The NPC is organized into four concentric rings:

The cytoplasmic ring and nuclear ring are each composed of eight spokes formed by the Y-complex (also called the Nup84 complex in yeast, Nup107-160 complex in vertebrates). Each Y-complex is a roughly 50 nm elongated assembly of 7-9 Nups arranged in a Y-shaped scaffold. The Y-complexes stack in two concentric rings per face (inner and outer), giving 16 Y-complexes per face and 32 per NPC. The Y-complex scaffold provides the architectural backbone: it is rigid, evolutionarily conserved from yeast to humans, and resistant to biochemical perturbation of the transport mechanism. The outer diameter of the scaffold ring is approximately 120 nm, and the inner diameter of the transport channel is approximately 40-55 nm.

The inner ring (also called the adaptor ring) sits between the cytoplasmic and nuclear rings and lines the central channel. It is composed of the Nup93 complex (Nup93, Nup188, Nup205 in vertebrates; Nic96, Nup192, Nup157 in yeast). The inner ring anchors the FG-repeat nucleoporins that form the transport barrier. The inner ring also provides the connection between the scaffold and the pore membrane — the curved region of the nuclear envelope where inner and outer membranes are fused.

The luminal ring sits within the perinuclear lumen (the space between inner and outer nuclear membranes) and braces the pore membrane. The Nups forming the luminal ring (Nup210/gp210 in vertebrates, Pom152/Pom34 in yeast) are transmembrane proteins that anchor the NPC in the nuclear envelope.

FG-repeat nucleoporins constitute the functional heart of the transport barrier. Roughly 10-12 different FG-Nups are positioned along the inner wall of the central channel and on the cytoplasmic filaments and nuclear basket. They are classified by their FG repeat type: GLFG repeats (found in Nup98, Nup153), FXFG repeats (found in Nup62, Nup58/45, Nup54, Nup153), and SGFG repeats. Each FG-Nup contains a structured anchoring domain and an intrinsically disordered region rich in FG repeats, extending flexible filaments 30-80 nm into the channel lumen. The density of FG repeats is roughly 3,000-5,000 per NPC.

Two principal models explain how the FG-Nup mesh functions as a selective barrier:

The selective phase model (Ribbeck and Gorlich, 2002) proposes that FG repeats engage in hydrophobic interactions with each other, forming a meshwork with properties analogous to a hydrogel. This meshwork excludes inert macromolecules by size and hydrophobicity. Transport receptors (karyopherins) have hydrophobic binding pockets that compete with inter-FG interactions, locally dissolving the mesh and allowing the receptor-cargo complex to partition through. The transport receptor acts as a surfactant, lowering the free energy barrier for passage through the hydrogel.

The virtual gating / reduction-of-dimensionality model (Rout et al., 2003) proposes that FG filaments form a dynamic, entropic barrier. In the absence of transport receptors, the FG filaments fluctuate freely, occupying the channel volume and presenting an entropic and steric barrier to large macromolecules. Transport receptors bind FG repeats, condensing the filaments and reducing their excluded volume, which opens a transient path for the receptor-cargo complex. The key insight is that transport receptors do not fight against the barrier — they bind to its components and thereby reduce the barrier locally.

Reconstituted NPC studies and single-molecule tracking suggest elements of both models contribute. FG-Nups can undergo phase separation in vitro, consistent with the selective phase model. However, the NPC channel is not a static gel — the FG filaments are highly dynamic, and single-molecule measurements show that transport receptors move through the channel by a series of rapid binding and unbinding events with effective diffusion coefficients of roughly 1-10 um^2/s within the channel, which is consistent with a virtual gating mechanism. The consensus emerging from cryo-ET and molecular dynamics simulations is that the FG mesh exists in a heterogeneous state: some regions form condensed hydrogel-like phases while others remain dynamic and entropic, and the transport receptor exploits both properties during transit.

Transport kinetics can be modeled as a Markov chain with the NPC channel divided into discrete binding sites. For a cargo-receptor complex moving through $n$ equivalent FG-Nup binding sites, the mean transit time is:

$$\tau =\frac{n}{k_{\text{on}}\cdot [\text{FG}]-k_{\text{off}}}$$

where $k_{\text{on}}$ is the association rate constant for the receptor-FG interaction, $[\text{FG}]$ is the effective concentration of FG repeats within the channel, and $k_{\text{off}}$ is the dissociation rate constant. Experimental measurements using single-molecule fluorescence give transit times of approximately 1-10 ms for importin-beta complexes through intact NPCs. The Ran-GTP gradient introduces asymmetry into this kinetic scheme: at the nuclear face of the NPC, Ran-GTP binding to importin-beta accelerates $k_{\text{off}}$ for the FG interaction, ensuring directional release.

The thermodynamic driving force for transport can be expressed as a free energy difference. For import of a cargo C by importin-beta (ImpB):

$$\Delta G_{\text{import}}=RT\ln \frac{[C{]}_{\text{nucleus}}/[C{]}_{\text{cytoplasm}}}{K_{\text{eq}}}$$

where $K_{\text{eq}}$ is set by the Ran-GTP gradient. At equilibrium, the distribution ratio of cargo across the NPC would be determined by the free energy of the Ran-GTP gradient. Because the Ran gradient is continuously regenerated (at the cost of GTP hydrolysis), the system operates far from equilibrium, allowing accumulation of cargo against a concentration gradient. Typical nuclear/cytoplasmic ratios for actively imported proteins exceed 100:1.

Topologically associated domains and three-dimensional genome organization

Beyond the binary euchromatin/heterochromatin distinction, the genome is organized into topologically associated domains (TADs) — contiguous chromatin regions (typically 0.1-1 Mb) within which DNA sequences preferentially interact. TADs were identified by Hi-C (chromosome conformation capture) and appear as self-interacting blocks on contact probability maps. TAD boundaries are enriched in CTCF binding sites, active transcription start sites, and housekeeping genes. The insulator protein CTCF, together with the cohesin complex, creates TAD boundaries through a loop extrusion mechanism: cohesin (a ring-shaped SMC complex) progressively extrudes chromatin through its lumen until it encounters convergently oriented CTCF sites, forming a stable loop. This mechanism partitions the genome into discrete regulatory neighborhoods — enhancers within a TAD preferentially contact promoters within the same TAD, while cross-TAD interactions are suppressed.

Lamina-associated domains (LADs) represent a distinct layer of spatial organization. LADs are identified by DamID (DNA adenine methyltransferase identification) using lamin B1 as bait. LAD boundaries frequently coincide with gene deserts, L1 LINE elements, and large chromosomal bands visible by Giemsa staining (G-bands). LADs are not static: during cellular differentiation, subsets of LADs detach from the periphery (becoming transcriptionally active) while other regions newly associate with the lamina (becoming silenced). This LAD switching correlates with lineage commitment — for example, during neurogenesis, neuronal genes leave LADs and become activated, while non-neuronal programs are recruited to the lamina and silenced. The molecular tethering mechanism involves the LINC complex adaptor protein SUN1, heterochromatin protein HP1, histone methyltransferases (EHMT1/GLP, EHMT2/G9a), and the chromatin-associated protein LBR (lamin B receptor).

Nucleoporins themselves participate in gene regulation beyond their structural role. A subset of Nups — particularly Nup98, Nup153, and Nup62 — can bind to active genes in the nuclear interior, away from the NPC. This "gene gating" phenomenon was first proposed by Blobel (1985) and has been validated by chromatin immunoprecipitation studies showing that Nup98 associates with developmentally regulated genes in Drosophila and mammalian cells, facilitating their rapid activation. NPCs may thus serve as scaffolds that organize both transport and gene expression.

Nuclear envelope breakdown and reassembly in mitosis

In higher eukaryotes, the nuclear envelope disassembles at the onset of mitosis (prometaphase) to allow spindle microtubules to access condensed chromosomes. NEBD proceeds through: (1) CDK1/cyclin B phosphorylation of lamins, triggering lamin depolymerization and nuclear lamina dissolution; (2) phosphorylation of Nups (particularly Nup98 and Nup153), causing NPC disassembly into subcomplexes; (3) membrane retraction — the nuclear envelope is absorbed into the ER, and NPC components are dispersed into the cytoplasm. Notably, a subset of Nups remain associated with kinetochores during mitosis, where they contribute to spindle assembly checkpoint signaling.

NEBD creates a transport crisis: the Ran gradient collapses because RCC1 is released from condensed chromatin and Ran is no longer segregated from RanGAP. Ran-GTP is generated locally around mitotic chromosomes by RCC1 that remains bound to chromatin, creating a Ran-GTP gradient around the mitotic spindle. This spatial gradient is essential for spindle assembly: Ran-GTP releases spindle assembly factors (SAFs) from importin-alpha/beta in the vicinity of chromosomes, promoting microtubule nucleation and stabilization near kinetochores while preventing inappropriate microtubule growth in distal cytoplasm.

Nuclear envelope reassembly in telophase is the reverse process: (1) BAF (barrier-to-autointegration factor) crosslinks chromatin with the LEM-domain proteins (emerin, LAP2, MAN1) on reforming membrane sheets; (2) membrane sheets derived from the ER enclose decondensing chromosomes; (3) NPC subcomplexes are recruited and assembled by a stepwise process (outer ring Y-complexes first, then inner ring, then FG-Nups); (4) lamin B is dephosphorylated by protein phosphatase 1 (PP1) and repolymerizes, re-establishing the nuclear lamina; (5) the Ran gradient re-forms as RCC1 re-associates with chromatin and the sealed envelope re-segregates nucleus from cytoplasm. The entire reassembly process takes approximately 10-20 minutes.

Nucleoporin diseases (nucleoporinopathies)

Mutations in nucleoporin genes cause a spectrum of human diseases. Triple A syndrome (Allgrove syndrome; achalasia, alacrima, adrenal insufficiency) is caused by mutations in AAAS encoding Aladin (ALADIN), a component of the NPC cytoplasmic ring. Aladin is a WD-repeat protein that anchors proteins involved in DNA repair (AP endonuclease, DNA polymerase beta) to the NPC. Loss of Aladin impairs the nuclear import of these repair enzymes, increasing oxidative DNA damage — particularly in tissues with high oxidative metabolism (adrenal glands, esophageal neurons, lacrimal glands), explaining the tissue-specific phenotype.

NUP214-ABL1 fusion is found in a subset of T-cell acute lymphoblastic leukemias (T-ALL). The fusion protein dimerizes constitutively, activating the ABL1 tyrosine kinase in a manner analogous to BCR-ABL in chronic myeloid leukemia. The NUP214 FG-repeat domain mediates the constitutive oligomerization.

NUP98 translocations are among the most common chromosomal rearrangements in acute myeloid leukemia (AML). The NUP98 FG-repeat domain is fused to various homeodomain or transcription factor partners (HOXA9, HOXD13, JARID1A, NSD1). The FG-repeat domain of NUP98 acts as a transcriptional activation domain in the fusion protein, driving aberrant expression of HOX genes and leukemogenic transcription programs.

Autoantibodies against Nups are diagnostic markers in systemic autoimmune diseases. Anti-gp210 antibodies are highly specific for primary biliary cholangitis (PBC), and anti-Nup62 antibodies are found in subsets of lupus and scleroderma patients.

Connections Master

Nuclear transport connects to nearly every cellular process that requires communication between the genome and the cytoplasm.

Gene expression 17.05.01. Transcription factors (NF-kB, p53, STATs, NF-AT, SMADs) are regulated by nucleocytoplasmic shuttling. In most cases, the transcription factor is synthesized in the cytoplasm and held inactive until a signal triggers its nuclear import via exposure or modification of an NLS. The export of mRNA through NXF1/TAP (a non-karyopherin pathway) couples transcription to translation: pre-mRNA splicing deposits the exon junction complex and TREX (transcription-export) complex, which recruits NXF1 for export. Disruption of mRNA export — as occurs in some viral infections (influenza NS1 protein blocks NXF1) — traps mRNA in the nucleus and shuts down host gene expression.

Cell cycle regulation. The master cell cycle regulator Cdc25 (a phosphatase that activates CDK1) shuttles between nucleus and cytoplasm via CRM1-dependent export. Cyclin B1 accumulates in the cytoplasm during G2 and floods into the nucleus at prophase when its NES is phosphorylated, triggering the positive feedback loop that drives mitotic entry. Nuclear envelope breakdown itself is triggered by CDK1/cyclin B1, which phosphorylates lamins and Nups — creating a circular dependency where nuclear transport enables the cell cycle transitions that eventually dismantle the transport system.

DNA repair. DNA double-strand break repair requires the coordinated nuclear import of repair factors. The MRN complex (Mre11-Rad50-Nbs1) assists in DSB sensing and recruits ATM. 53BP1 and BRCA1 are imported via the importin pathway. The nuclear pore itself participates in repair: in yeast, persistent DSBs are tethered to the NPC via the Nup84 complex (a component of the Y-complex), which facilitates repair by both non-homologous end joining (NHEJ) and homologous recombination (HR). This NPC-anchored repair pathway is conserved in Drosophila and mammalian cells, where Nup153 and Tpr recruit SUMO proteases and repair factors to persistent breaks.

Signal transduction. The NF-kB pathway provides the paradigm for signal-dependent nuclear import (see the worked example above). Other examples include the Wnt pathway (beta-catenin is stabilized in the cytoplasm upon Wnt signaling and enters the nucleus to activate target genes — notably, beta-catenin does not use the classical importin pathway but interacts directly with FG-Nups), the JAK-STAT pathway (phosphorylated STAT dimers expose an NLS and are imported by importin-alpha/beta), and the TGF-beta pathway (phosphorylated SMAD complexes are imported and then exported by CRM1 after signaling terminates).

Mechanics and aging. Laminopathies — diseases caused by mutations in LMNA or genes encoding lamin-associated proteins — reveal the mechanical role of the nuclear lamina. Beyond HGPS (see Exercise 6), LMNA mutations cause Emery-Dreifuss muscular dystrophy, dilated cardiomyopathy, familial partial lipodystrophy, and Charcot-Marie-Tooth disease type 2B1. The tissue specificity of different LMNA mutations remains an active research question, but the emerging model is that tissues experiencing high mechanical load (skeletal muscle, cardiac muscle, adipose tissue, peripheral nerves) are most sensitive to lamina defects because their nuclei undergo repeated deformation cycles that require an intact lamina to prevent rupture and DNA damage.

Historical notes Master

The existence of the nuclear envelope was established by light microscopy in the 19th century, but the nuclear pore complex was not visualized until the advent of transmission electron microscopy in the 1950s. Callan and Tomlin (1950) first described "pores" in the nuclear envelope of amphibian oocytes, noting their regular spacing. Gall (1967) provided detailed ultrastructural descriptions of NPC architecture using thin-section EM, revealing the eight-fold symmetry and the central granule (now understood to be a translocating transport complex, not a structural component).

The field of nuclear transport was transformed by the discovery of the nuclear localization signal. De Robertis and colleagues (1978) showed that injection of SV40 viral DNA into frog oocytes led to nuclear accumulation of the large T antigen. Dingwall, Laskey, and colleagues (1982) demonstrated that a short basic sequence (PKKKRKV) was necessary and sufficient for nuclear import, establishing the NLS concept. Goldfarb, Roberts, and colleagues (1986) identified the bipartite NLS in nucleoplasmin.

The molecular machinery was identified through a combination of biochemical reconstitution and genetic approaches. Adam and Adam (1994) developed the digitonin-permeabilized cell assay, which allowed identification of the soluble factors required for import: importin-alpha, importin-beta, Ran, and NTF2. Gorlich, Prehn, Laskey, and Hartmann (1994) isolated importin-beta as the transport receptor. Fornerod, Ohno, and colleagues (1997) identified CRM1 as the exportin for leucine-rich NES-bearing proteins, and the specific inhibitor leptomycin B was shown to target CRM1 by Kudo, Nishi, and colleagues (1998).

Ran was discovered as a Ras-like GTPase (Ras-related nuclear protein) by Bischoff and Ponstingl (1991). The critical role of the Ran gradient was established by Gorlich, Panté, and colleagues (1996), who showed that Ran-GTP promotes import complex disassembly in the nucleus. The asymmetric localization of RCC1 (nuclear) and RanGAP (cytoplasmic) was demonstrated by Ohtsubo, Okazaki, and Nishimoto (1989) and Bischoff, Klebe, and colleagues (1995).

FG-repeat nucleoporins were first characterized by Davis (Davis and Blobel, 1986; Davis, 1995), who showed that O-linked N-acetylglucosamine (a sugar modification found almost exclusively on nucleoporins) decorated proteins containing FG repeats. Rout and Wente (1994) proposed that FG-Nups form the selective barrier. The two competing models for the barrier mechanism — the selective phase model (Ribbeck and Gorlich, 2002) and the virtual gating model (Rout et al., 2003) — emerged from in vitro reconstitution studies and biophysical modeling, respectively.

Chromatin organization progressed from the binary euchromatin/heterochromatin distinction (Heitz, 1928, based on cytological staining) to the modern three-dimensional genome view. Hi-C technology (Lieberman-Aiden et al., 2009) revealed TADs as a fundamental organizing principle. LADs were mapped genome-wide by Guelen et al. (2008) using DamID with lamin B1. The loop extrusion model for TAD formation was proposed independently by Fudenberg, Imakaev, and colleagues (2016) and Sanborn, Rao, and colleagues (2015), and was experimentally validated by single-molecule imaging of cohesin translocating along DNA (Golfier, Quail, and colleagues, 2020).

The cryo-EM structure of the human NPC was resolved at near-atomic resolution by two groups simultaneously in 2022: Mosalaganti, Kosinski, and colleagues (EMBL) and Schuller, Appel, and colleagues (MPI), both achieving approximately 10-15 Angstrom resolution for the scaffold and higher resolution for individual subcomplexes. These structures confirmed the eight-fold symmetric architecture and provided the first complete inventory of the human NPC proteome.

Bibliography Master

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