| RanBP2 is a large and unique vertebrate protein comprised of several structural domains. RanBP2 associates with the RanGTPase, a master regulatory GTPase, which modulates nucleocytoplasmic trafficking. RanBP2 was also localized at cytoplasmic fibrils emanating from nuclear pore complexes of liver and HeLa cells. These data, interaction of RanBP2 with the nuclear import and export receptors, and inhibition of RanBP2-dependent nuclear import of cargoes, implicated a role of RanBP2 in nucleocytoplasmic trafficking. However, RanBP2 was identified also in retinal neurons, where it is abundant, and its subcellular distribution apparently departs, in part, from that observed in other cells. For example, RanBP2 is abundant in the mitochondria-rich ellipsoid compartment of the sensory neurons and at restricted foci along cytoplasmic tracks of ganglion cells. In addition, RanBP2 enhances in vivo the production of the light-receptor, opsin. These data suggest additional roles of RanBP2 in cell function besides modulating nucleocytoplasmic trafficking. The goal of this thesis was to determine further roles of RanBP2 in cell function. To this end, I identified two novel molecular partners with binding activity towards specific domains of RanBP2. The components of the 19S cap of the 26S proteasome associate with the cyclophilin-like domain (CLD) in a neuronal-specific fashion. A novel domain between RBD2 and RBD3 of RanBP2 (JX2) mediates the specific interaction with a subset of microtubule-based kinesin motor proteins, KIF5B and KIF5C. An amino acid conserved in KIF5BIKIF5C, but not KIF5A, was determined to underlie the KIF5-isotype specificity towards RanBP2. Moreover, we recently found another domain of RanBP2 mediates its interaction with the mitochondrial chaperone, Cox11, and they colocalize with mitochondria throughout the perikarya. Altogether, these data suggest RanBP2 serves as a scaffold chaperone for kinesin-mediated transport of a variety of cellular cargoes. In support of this, specific disruption of the interaction between RanBP2 and, KlF5B and KIF5C, leads to drastic perinuclear clustering of RanBP2 and mitochondria followed by extremely severe cellular atrophy. Gene-dosage studies indicate RanBP2 is vital for cell function and it affects energy homeostasis in mice. Altogether, this work directly links RanBP2 to the transport and/or biogenesis of mitochondria, a process likely to be tightly coupled to the transport of other protein machinery. Hence, we have identified a vital trafficking pathway with physiological implications in human disease and unique to vertebrates. |
