SnapShot: Elizabeth J. Tran, Timothy A. Bolger, and Susan R. Wente Cell and Developmental Biology, Vanderbilt University Medical Center, Nashville, TN 37232, USA

S. cerevisiae Metazoan ­Transport Karyopherins Complex & Other & Other Category Receptors Cargo(s) Essential Receptors Cargo(s)

I Kap95 Kap60 adaptor Yes -b1 Importin-a adaptor for cNLS for cNLS proteins, Snurportin adaptor for U snRNPs

II Kap95 — Yes Importin-b1 SREBP-2, HIV Rev and TAT, Cyclin B

Kap104 Nab2, Hrp1 t.s. Transportin or PY-NLS-containing (mRNA-binding Transportin 2 proteins, mRNA- proteins) binding proteins, histones, ribosomal proteins

Kap123 SRP proteins, No Importin 4 histones, ribosomal histones, ribo- S3a, somal proteins Transition Protein 2

Kap111/Mtr10 Npl3 (mRNA- t.s. Transportin SR1 SR proteins, HuR binding protein), or SR2 tRNAs

Kap121/Pse1 ribosomal Yes Importin 5/ histones, ribosomal proteins, Yra1, Importin-b3/ proteins

Import Spo12, Ste12, RanBP5 Yap1, Pho4, histones

Kap119/Nmd5 ribosomal pro- No Importin 7 GR, ribosomal pro- teins, histones, teins, Smad, ERK Hog1, Crz1, Dst1

Kap108/Sxm1 Lhp1, ribosomal No Importin 8 SRP19, Smad proteins

Kap114 TBP, histones, No Importin 9 histones, ribosomal Nap1, Sua7 proteins

Kap122/Pdr6 Toa1 and Toa2, No — — TFIIA

Kap120 Rpf1 No — —

— — — Importin 11 UbcM2, rpL12

Ntf2 /Gsp1 (GDP- Yes NTF2 Ran (GDP-bound bound form) form)

III Kap142/Msn5 Replication pro- No — — tein A (import), Pho4, Crz1, Export Cdh1 (export)

— — — Importin 13 UBC9, Y14 (import),

Import/ ­ eIFIA (export)

IV Crm1/Xpo1 -rich NES Yes CRM1/ ­ Leucine-rich NES sequences Exportin 1 sequences

Kap109/Cse1 Kap60/Srp1 Yes CAS Importin-a family members

Kap127/Los1 tRNAs No Exportin-t tRNAs

— — — Exportin 4 eIF5A

— — — Exportin 5 pre-miRNA

— — — Exportin 6 Profilin,

— — — RanBP16/ p50-RhoGAP Exportin 7

V Crm1/Xpo1 Nmd3 adaptor Yes CRM1/ NMD3 adaptor

Export for 60S ribosomal Exportin 1 for 60S ribosomal subunit subunit

Mex67-Mtr2 60S via 5S rRNA Yes — —

Arx1 60S ribosomal No — — subunit

Crm1/Xpo1 Ltv1 putative Yes CRM1/ 40S ribosomal adaptor for Exportin 1 subunit 40S ribosomal subunit

VI Mex67-Mtr2 Npl3, Yra1, and Yes TAP/NXF1-p15/ REF/Aly adaptor for Hpr1 adaptors NXT1 mRNA for mRNA

420 Cell 131, October 19, 2007 ©2007 Elsevier Inc. DOI 10.1016/j.cell.2007.10.015 See online version for legend, references, and abbreviations.. SnapShot: Nuclear Transport Elizabeth J. Tran, Timothy A. Bolger, and Susan R. Wente Cell and Developmental Biology, Vanderbilt University Medical Center, Nashville, TN 37232, USA

Exchange of macromolecules between the nucleus and the cytoplasm occurs through complexes (NPCs), large proteinaceous structures embedded within the nuclear envelope. Small molecules (<30 kDa) can passively diffuse through NPCs. Nuclear transport receptors are required to facilitate import and export of large molecules (>30 kDa). This includes the -β (Kapβ) receptor family wherein each associates directly with the NPC and has cargo binding controlled by the asymmetric distribution of Ran-GTP and the Ran cycle (see Figure). Nuclear localization of the Ran guanine-nucleotide exchange factor (Ran-GEF, Prp20 in S. cerevisiae) via chromatin association maintains high concentrations of Ran-GTP in the nucleus. Conversely, Ran-GDP predominates in the cytoplasm through the com- bined action of cytoplasmically localized Ran-Binding Protein 1 (RanBP1, Yrb1 in S. cerevisiae) and Ran GTPase–Activating Protein (Ran-GAP, Rna1 in S. cerevisiae). During import, Kapβ receptors interact with cytoplasmic cargos and traverse the NPC. In the nucleus, Ran-GTP binds to the importing Kapβ and triggers cargo release. Conversely, during export, Ran-GTP stabilizes the association of Kapβ receptors with nuclear cargo. Hydrolysis of GTP to GDP at the cytoplasmic NPC face promotes disassembly of the export receptor-cargo complex. Transport receptors are categorized based on transport direction, the use of cargo adaptors, and transport of spe- cialized cargo such as Ran-GDP, , and/or mRNA. (I) Import receptor complexes with adaptors: Kap60-Kap95 (importin-α-importin-β in metazoa) is the prototypical import receptor. Cargo with a classical nuclear local- ization signal (cNLS) is recognized by the adaptor Kap60, which binds the receptor Kap95. Whereas yeast has only one importin-α, metazoans have several importin-α homologs (not shown). (II) Import receptors that bind cargo directly: Many Kapβs, including Kap104, can bind directly to their cargo. (III) Bidirectional transport receptors: The receptor Msn5 has the striking capacity to function in both import and export. (IV) Export receptor complexes: The first identified export receptor, Crm1, recognizes leucine-rich nuclear export sequences (NES) in cargo proteins. (V) Export of ribosomal subunits: Large cargos, such as ribosomal subunits, require multiple receptors for transport. Export of the assembled 60S subunit requires Crm1 (with the adaptor Nmd3) and, in S. cerevisiae, the Mex67-Mtr2 heterodimer as well as Arx1. Export of the 40S subunit also requires Crm1 and possibly an adap- tor, Ltv1. (VI) Export of mRNA: Export of most mRNA is Ran independent and, instead, requires the nonkaryopherin Mex67-Mtr2 (TAP/NXF1 and p15/NXT1 in metazoans, re- spectively). Mex67-Mtr2 binds mRNA thorough association with mRNA-binding proteins, or some metazoan mRNAs can recruit TAP/NXF1 by directly binding a specific RNA element, the CTE (not shown). CTEs are also present in some viral . Directionality of mRNA export is determined by the ATPase Dbp5, whose activation is

controlled by spatially restricted inositol hexakisphosphate (IP6)-bound Gle1. Consensus Sequences

cNLS: PKKKRKV (monopartite) or KRX(10–12)KRRK (bipartite); PY-NLS: ϕ(G/S/A)ϕϕX(5–10)(R/H/K)X(2–5)PY (hPY-NLS) or basic-enriched(5–8)X(2–7) (R/H/K)X(2–5)PY (bPY-NLS), ϕ denotes hydrophobic residue; NES: (L/V)X(2–3)(ψ/F)X(2–3)LXψ, ψ denotes large aliphatic residue (I, V, L, M). Abbreviations NLS, nuclear localization signal; NES, ; t.s., temperature sensitive; U snRNP, uridine-rich small nuclear ribonucleoprotein complex; SR, serine/ar- ginine-rich; TBP, TATA-binding protein; SRP, signal recognition particle; hnRNP, heterogeneous nuclear ribonucleoprotein.

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420.e1 Cell 131, October 19, 2007 ©2007 Elsevier Inc. DOI 10.1016/j.cell.2007.10.015