Abstract
The Ran-binding protein 2 (RanBP2) is a large multimodular and pleiotropic protein. Several molecular partners with distinct functions interacting specifically with selective modules of RanBP2 have been identified. Yet, the significance of these interactions with RanBP2 and the genetic and physiological role(s) of RanBP2 in a whole-animal model remain elusive. Here, we report the identification of two novel partners of RanBP2 and a novel physiological role of RanBP2 in a mouse model. RanBP2 associates in vitro and in vivo and colocalizes with the mitochondrial metallochaperone, Cox11, and the pacemaker of glycolysis, hexokinase type I (HKI) via its leucine-rich domain. The leucine-rich domain of RanBP2 also exhibits strong chaperone activity toward intermediate and mature folding species of Cox11 supporting a chaperone role of RanBP2 in the cytosol during Cox11 biogenesis. Cox11 partially colocalizes with HKI, thus supporting additional and distinct roles in cell function. Cox11 is a strong inhibitor of HKI, and RanBP2 suppresses the inhibitory activity of Cox11 over HKI. To probe the physiological role of RanBP2 and its role in HKI function, a mouse model harboring a genetically disrupted RanBP2 locus was generated. RanBP2−/− are embryonically lethal, and haploinsufficiency of RanBP2 in an inbred strain causes a pronounced decrease of HKI and ATP levels selectively in the central nervous system. Inbred RanBP2+/− mice also exhibit deficits in growth rates and glucose catabolism without impairment of glucose uptake and gluconeogenesis. These phenotypes are accompanied by a decrease in the electrophysiological responses of photosensory and postreceptoral neurons. Hence, RanBP2 and its partners emerge as critical modulators of neuronal HKI, glucose catabolism, energy homeostasis, and targets for metabolic, aging disorders and allied neuropathies.
Highlights
The Ran-binding protein 2 (RanBP2)/Nup358 is a unique vertebrate and large scaffold protein comprised of multiple structural and functional domains [1,2,3,4]
The leucine-rich domain (LD) of RanBP2 Interacts with Cox11 and hexokinase type I (HKI) The LD of RanBP2 (Figure 1A) is a large and orphan domain of ;700 residues (;80 kDa), for which no molecular partners have been identified until this date
We extended the subcellular colocalization studies on RanBP2 and its novel partners by immunocytochemistry to determine if RanBP2, Cox11, HKI, and mHsp70 colocalize in hippocampal neurons (Figure 3A–3C), cerebral cortex neurons (Figure 3D–3F), ellipsoid subcellular compartments of photosensory neurons of the retina (Figure 3G–3O), and dissociated primary glia and neuron cultures from the brain (Figure 3P–3Z)
Summary
The RanBP2/Nup358 is a unique vertebrate and large scaffold protein comprised of multiple structural and functional domains [1,2,3,4]. Several roles of RanBP2 have emerged that implicate RanBP2 in nucleocytoplasmic trafficking [3,5], protein biogenesis [6,7], the formation of the mitotic spindle, assembly of the nuclear envelope [8], and the integration of the nuclear envelope breakdown with kinetochore formation and maturation during early mitotic progression [9]. The specific interaction of RanBP2 with a diverse set of partners likely reflects a pleiotropic role of RanBP2 in cell function, possibly through the integration of multiple pathways. The Ran-binding domains RBDn=1-4 of RanBP2 associate with the nuclear import co-receptor, importin-b [10,11], and antibodies against RanBP2 inhibit the nuclear import pathway in HeLa cells [3]; but such a role seems dispensable in Xenopus oocytes [12]. The combination of the C-terminal domains, RBD4 and CY (of RanBP2), associates with a subset of G protein-coupled receptors, the red/green opsin, expressed in photosensory neurons and enhances opsin functional production [6,7], while the interaction of the KBD of RanBP2 with a subset of the conventional microtubule-based motor proteins, the kinesins, KIF5B and KIF5C, occurs selectively in the central nervous system (CNS) [13]
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