Structural Biology and Regulation of Protein Import into the Nucleus

Abstract

Proteins are translated in the cytoplasm, but many need to access the nucleus to perform their functions. Understanding how these nuclear proteins are transported through the nuclear envelope and how the import processes are regulated is therefore an important aspect of understanding cell function. Structural biology has played a key role in understanding the molecular events during the transport processes and their regulation, including the recognition of nuclear targeting signals by the corresponding receptors. Here, we review the structural basis of the principal nuclear import pathways and the molecular basis of their regulation. The pathways involve transport factors that are members of the β-karyopherin family, which can bind cargo directly (e.g., importin-β, transportin-1, transportin-3, importin-13) or through adaptor proteins (e.g., importin-α, snurportin-1, symportin-1), as well as unrelated transport factors such as Hikeshi, involved in the transport of heat-shock proteins, and NTF2, involved in the transport of RanGDP. Solenoid proteins feature prominently in these pathways. Nuclear transport factors recognize nuclear targeting signals on the cargo proteins, including the classical nuclear localization signals, recognized by the adaptor importin-α, and the PY nuclear localization signals, recognized by transportin-1. Post-translational modifications, particularly phosphorylation, constitute key regulatory mechanisms operating in these pathways.

Importin-β and CRM1 control a RANBP2 spatiotemporal switch essential for mitotic kinetochore function

“…In structural studies, importin-β interacts with RANBP2 via a NUP-binding domain in the N-terminal region, and FG-rich regions in RANBP2 (reviewed by Bednenko et al, 2003;Christie et al, 2016). CRM1 -together with RANGTPinteracts with RANGAP1 (Arnaoutov et al, 2005;Wu et al, 2013).…”

“…RANBP2 is the most cytoplasmic NUP, and interacts with importin-β in the initial steps of the import cycle, i.e. when import complexes assembled in the cytoplasm dock to the NPC to traverse it (Bednenko et al, 2003;Christie et al, 2016). After NE breakdown, importin-β associates with mitotic MTs (Ciciarello et al, 2004) and interacts with spindle-associated and MT-regulatory factors.…”

Comparative interactomics provides evidence for functional specialization of the nuclear pore complex

“…44,45 We also know that there are 2 major kinds of transport factor: karyopherin-like, and nonkaryopherin-like, with very different preferences for FG repeat flavors. 46,47 Certainly, trypanosomes show us that FG flavor may not necessarily be all about nucleocytoplasmic positioning or imparting a directionality to nuclear transport. Perhaps instead, these 2 major FG repeat flavors delineate specific transport conduits for the trafficking pathways across the NPC mediated by the 2 kinds of transport factor?…”

RNA-binding protein HuR regulates nuclear import of protein

“…Consequently, holo-CRABP2 is recognized by importin-α1 (encoded for by Kpna2), enabling its ligand-induced nuclear translocation (Sessler and Noy, 2005). Other importin-β proteins, such as transportin 1/importin-β2 (encoded for by Tnpo1), directly bind cargoes that contain a broad range of 'non-classical' NLSs characterized by a Cterminal sequence comprised of Arg separated from a Pro-Tyr (PY) motif by two to five residues (Christie et al, 2015). An additional key component of the nuclear import machinery is the small GTPase Ran.…”

“…GTP hydrolysis by Ran is also stimulated by the Ran-binding protein RanBP1 (Bischoff et al, 1995). Nuclear RanGEF and cytosolic RanGAP/ RanBP1 thus form a RanGDP/GTP gradient that allows cargo-loaded importins to release their cargo in the nucleus and to dissociate from Ran in the cytosol (reviewed in Christie et al, 2015).…”