#importin

Protein Sorting and Trafficking

What role does NTF2 play in nuclear transport?

Maintenance of steep Ran gradient requires continuous recycling of Ran between compartments after hydrolysis. Ran-GDP formed in cytoplasm diffuses poorly across pore and requires facilitated import. Nuclear transport factor 2, small 14 kilodalton homodimer forming beta-sandwich barrel with hydrophobic cavity, specifically binds Ran-GDP conformation with high affinity but excludes Ran-GTP because switch I and switch II regions adopt different positions blocking NTF2 interaction. NTF2-Ran-GDP complex interacts transiently with FG repeats of central nucleoporins Nup62, Nup58 and Nup54 via surface

Ref: Stewart M et al., J Cell Biol 147: 1999, NTF2 Imports Ran-GDP into Nucleus.

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Protein Sorting and Trafficking

Which of the following processes is not directly dependent on Ran-GTP?

Most nuclear import of NLS proteins using importin-alpha-beta heterodimer and export of NES proteins via CRM1 strictly consume Ran-GTP cycles, with RCC1 generating Ran-GTP inside nucleus and RanGAP1 hydrolyzing outside. Each cycle consumes one GTP and relies on NTF2 recycling of Ran-GDP. Messenger RNA departure follows divergent energetic logic. Bulk poly(A) mRNA transcribed, spliced and polyadenylated recruits TREX complex and adaptor Aly/REF that loads heterodimer NXF1-NXT1 which binds FG repeats directly without Ran. At cytoplasmic filaments of pore, DEAD-box helicase Dbp5/DDX19 activated b

Ref: Stewart M, Nature Rev Mol Cell Biol 8: 2007, Ran-Dependent Protein Transport versus Ran-Independent mRNA Export.

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Protein Sorting and Trafficking

Which protein is responsible for releasing cargo from the importin receptor in the nucleus?

Efficient nuclear import demands timely liberation of cargo after arrival inside nucleoplasm so it can engage chromatin, transcription machinery or splicing. Importin-beta-cargo interaction stable in cytoplasm where Ran-GTP scarce becomes unstable upon encountering high nuclear Ran-GTP concentration. Ran-GTP docks onto N-terminal arch of importin-beta composed of HEAT repeats, inducing large allosteric opening that widens superhelical curvature and disrupts binding interface for importin-alpha IBB domain and for FG nucleoporins. Importin-alpha affinity for basic NLS drops, cargo released aided

Ref: Vetter et al., Nature 392: 1998, RanGTP Dissociates Import Complexes in Nucleus.

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Protein Sorting and Trafficking

What ensures directionality in nuclear protein transport?

Unidirectional accumulation of proteins against concentration gradients through nuclear pores cannot arise from gate selectivity alone because FG nucleoporin mesh permits diffusion in both directions. Direction emerges from asymmetric distribution of Ran regulators generating chemical potential. Nuclear RCC1 bound to chromatin continuously exchanges GDP for GTP using nuclear GTP pool, while cytoplasmic RanGAP1 SUMOylated and tethered to RanBP2/Nup358 plus soluble RanBP1 hydrolyzes GTP outside, creating gradient high inside low outside, more than hundred-fold difference. Importins bind NLS carg

Ref: Nature Reviews Mol Cell Biol 20: 2019, Ran GTPase Gradient as Directionality Determinant in Nuclear Transport.

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Protein Sorting and Trafficking

What is the role of importins in nuclear transport?

Importins act as soluble chaperones that decode nuclear localization signals and ferry cargo through nuclear pore complex while keeping hydrophobic patches shielded. Classical monopartite NLS such as SV40 large T antigen PKKKRKV or bipartite nucleoplasmin KRPAATKKAGQAKKKK are highly enriched in basic lysine and arginine side chains that bind armadillo repeats of importin-alpha via electrostatic interactions and tryptophan-mediated cation-pi stacking. Importin-beta then wraps around importin-alpha IBB domain and mediates transient hydrophobic contacts with FG nucleoporins, moving complex inward

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: Importin Alpha-Beta and NLS Recognition Mechanism.

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Protein Sorting and Trafficking

Which GTPase is specifically involved in nuclear transport?

Ran is small 25 kilodalton Ras-family GTPase that specializes in marking nuclear versus cytoplasmic compartments, unlike Rab, Rho and Ras which control vesicle docking, actin remodeling and MAP kinase signaling elsewhere. Its nucleotide state differs across envelope because regulators are spatially segregated: RCC1 guanine nucleotide exchange factor bound to histones H2A-H2B inside nucleus generates high Ran-GTP, while RanGAP1 SUMOylated and anchored to RanBP2/Nup358 at cytoplasmic filaments together with RanBP1 accelerate GTP hydrolysis outside producing Ran-GDP. This partitioning creates ste

Ref: Alberts et al., Molecular Biology of the Cell, 6th ed., Chapter 12: Ran GTPase Gradient, RCC1 and RanGAP.

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Protein Sorting and Trafficking

The export of proteins from the nucleus requires:

Export of proteins from the nucleus depends on recognition of leucine-rich nuclear export signals that provide dominant cue for exit to the cytoplasm. Canonical NES consists of short amphipathic alpha-helix with regularly spaced large hydrophobic residues, typically leucine, isoleucine, valine or phenylalanine, arranged in pattern Φ-X2-3-Φ-X2-3-Φ-X-Φ where Φ denotes hydrophobic. This motif docks into hydrophobic groove formed by HEAT repeats 11 and 12 of chromosome region maintenance 1, CRM1, also called Exportin-1 or XPO1, stabilized only when Ran is in GTP-bound state present at high concent

Ref: Lodish et al., Molecular Cell Biology, 9th ed., Chapter 13: CRM1 Exportin Recognition of Leucine-Rich NES.

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