The Docking of Kinesins, KIF5B and KIF5C, to Ran-binding Protein 2 (RanBP2) Is Mediated via a Novel RanBP2 Domain

Abstract

The Ran-binding protein 2 (RanBP2) is a vertebrate mosaic protein composed of four interspersed RanGTPase binding domains (RBDs), a variable and speciesspecific zinc finger cluster domain, leucine-rich, cyclophilin, and cyclophilin-like (CLD) domains. Functional mapping of RanBP2 showed that the domains, zinc finger and CLD, between RBD1 and RBD2, and RBD3 and RBD4, respectively, associate specifically with the nuclear export receptor, CRM1/exportin-1, and components of the 19 S regulatory particle of the 26 S proteasome. Now, we report the mapping of a novel RanBP2 domain located between RBD2 and RBD3, which is also conserved in the partially duplicated isoform RanBP2L1. Yet, this domain leads to the neuronal association of only RanBP2 with two kinesin microtubulebased motor proteins, KIF5B and KIF5C. These kinesins associate directly in vitro and in vivo with RanBP2. Moreover, the kinesin light chain and RanGTPase are part of this RanBP2 macroassembly complex. These data provide evidence of a specific docking site in RanBP2 for KIF5B and KIF5C. A model emerges whereby RanBP2 acts as a selective signal integrator of nuclear and cytoplasmic trafficking pathways in neurons.The small nuclear GTPase, Ran, is a key regulator of protein (1-3) and RNA (4 -7) nuclear export and protein nuclear import (2, 6, 8 -11). In contrast to earlier proposals (12, 13) and other GTPase-mediated processes (14), recent data support that nucleocytoplasmic trafficking of cargoes across the nuclear envelope is independent of nucleotide hydrolysis (15-21). Instead, a predicted Ran-GTP to Ran-GDP gradient from the nucleus to the cytosol is proposed to propel the nuclear transport across the nuclear pore in a vectorial fashion (2,22,23). This predicted Ran-nucleotide gradient, together with the selective compartmentalization of key RanGTPase modulators in the nucleus and cytosol, limits the pool of transporters available for polarized delivery of cargoes into and from the nucleus, as these carriers act as sensors of the nucleotide-bound state of Ran (for review see Refs. 24 and 25). Among several Ran-dependent nuclear transporters recently identified, CRM1/exportin-1 (26), a member of the importin ␤/karyopherin-␤ class (22, 23), was shown to associate with Ran-GTP and mediate the nuclear export of substrates containing nuclear export signal sequences (27-31). Three key players with restricted subcellular compartmentalization are thought to play a pivotal role in maintaining the predicted Ran-nucleotide bound gradient across the nuclear envelope. The chromatin-associated Ran-nucleotide exchange factor, RCC1 (32), promotes the production of nuclear Ran-GTP via the exchange of RanGDP to RanGTP (33). In the cytosol, RanGTP hydrolysis seems to be mediated by the costimulation of RanGTPase-activating protein (34) and high affinity 35). This mechanism presumably ensures that loading of cargo destined for nuclear import and unloading of nuclear exported substrates, and loading of cargo for nuclear export and unloading of nuclear imported cargo, re...

A Novel Interaction between Kinesin and p120 Modulates p120 Localization and Function

“…Kinesin associates with RanBP2, an important regulator of the nuclear export complex (33). RanBP2 also interacts with CRM1p (exportin1), which binds nuclear proteins via their NES domains and promotes their nuclear export.…”

Conventional Kinesin Holoenzymes Are Composed of Heavy and Light Chain Homodimers

“…Specifically, three kinesin-1 genes [kinesin-1A, kinesin-1B, and kinesin-1C, formerly known as KIF5A, -B, and -C (7)] and two KLC genes [KLC1 and KLC2 (24)] have been identified in mammalian nervous tissue. Although the biological significance of this heterogeneity in conventional kinesin subunits is unknown, it might play a role in the selective targeting of conventional kinesin to different cargoes (13) and in the differential regulation of their transport by effector proteins (25). Earlier studies provided partial information on the interaction among selected subunits of conventional kinesin (24,26,27).…”

Nuclear pore complex tethers to the cytoskeleton

“…It was discovered that in retina and brain, the kinesin heavy chain microtubule motors, KIF5B and KIF5C, bind to Nup358 at a specific domain (between Ran binding domains 2 and 3) (Cai et al, 2001). It was suggested that the kinesin plus end directed motor could pay a role in delivering cargo to, or taking it away from, the NPC (Mavlyutov et al 2002), a function that may be more crucial in cells with long distance transport requirements, such as neurons.…”