Cyclophilins are ubiquitous and abundant proteins that exhibit peptidyl prolyl cis-trans isomerization (PPlase) activity in vitro. Their functions in vivo, however, are not well understood. Two new retinal cyclophilin isoforms, types I and II, are highly expressed in cone photoreceptors of the vertebrate retina. Type-II cyclophilin is identical to RanBP2, a large protein that binds the GTPase Ran. Here we report that two contiguous domains in RanBP2, Ran-binding domain 4 (RBD4) and cyclophilin, act in concert as a chaperone for the opsin molecule of the red/green-sensitive visual pigment of a dichromatic vertebrate. In Drosophila, the cyclophilin NinaA is expressed in all photoreceptors and is required for the expression of only a subset of opsins. The molecular basis of these photoreceptor class-specific effects and the functions of NinaA and other cyclophilins in vivo remain unclear. Unlike NinaA, which forms a stable complex with opsin from retinular cells R1-6, we find that the cyclophilin domain of RanBP2 does not bind opsin directly; rather, it augments and stabilizes the interaction between red/green (R/G) opsin and the RBD4 domain. This involves a cyclophilin-mediated modification of R/G opsin, possibly involving proline isomerization. The RBD4-cyclophilin supradomain of RanBP2, therefore, is a form of vertebrate chaperone of defined substrate specificity, which may be involved in the processing and/or transport of long-wavelength opsin in cone photoreceptor cells.

Background: Cyclophilins harbor ill-defined chaperone and prolyl isomerase activities toward physiological substrates. Results: Nonoverlapping chaperone or prolyl isomerase activity loss of Ran-binding protein 2 (Ranbp2) cyclophilin domain triggers unique impairments of proteostasis in distinct cell types and ubiquitin-proteasome system. Conclusion: Ranbp2 cyclophilin subdomains present discriminating physiological activities toward substrates or regulation of ubiquitin-proteasome system. Significance: Ranbp2-mediated mechanistic links in proteostasis with physiological and therapeutic relevance are uncovered.The immunophilins, cyclophilins, catalyze peptidyl cis-trans prolyl-isomerization (PPIase), a rate-limiting step in protein folding and a conformational switch in protein function. Cyclophilins are also chaperones. Noncatalytic mutations affecting the only cyclophilins with known but distinct physiological substrates, the Drosophila NinaA and its mammalian homolog, cyclophilin-B, impair opsin biogenesis and cause osteogenesis imperfecta, respectively. However, the physiological roles and substrates of most cyclophilins remain unknown. It is also unclear if PPIase and chaperone activities reflect distinct cyclophilin properties. To elucidate the physiological idiosyncrasy stemming from potential cyclophilin functions, we generated mice lacking endogenous Ran-binding protein-2 (Ranbp2) and expressing bacterial artificial chromosomes of Ranbp2 with impaired C-terminal chaperone and with (Tg-Ranbp2 WT-HA ) or without PPIase activities (Tg-Ranbp2 R2944A-HA ). The transgenic lines exhibit unique effects in proteostasis. Either line presents selective deficits in M-opsin biogenesis with its accumulation and aggregation in cone photoreceptors but without proteostatic impairment of two novel Ranbp2 cyclophilin partners, the cytokine-responsive effectors, STAT3/STAT5. Peptidyl cis-trans-prolyl isomerization is a rate-limiting step in protein folding (1-3). The catalysis of the cis-trans interconversion of the peptidyl-prolyl isomers is catalyzed by peptidylprolyl cis-trans isomerases (PPIase) 5 (4 -6). PPIases compose three families of structurally unrelated proteins, the cyclophilins (CyP), FK506-binding proteins (FKBP), and parvulins (7). * This work was supported, in whole or in part, by National Institutes of Health Grants EY019492, GM083165, and GM083165-03S1 (to P. A. F.), 2P30-EY005722 (to Duke University Eye Center), and 5P30NS061789 (to Duke Neurotransgenic Laboratory). This work was also supported by the Foundation Fighting CyPs and FKBPs are designated also as immunophilins, because they mediate immunosuppression (8,9). This effect is achieved by a gain-of-function mechanism upon binding of the immunosuppressive metabolites, cyclosporin A (CsA) or FK506, to the PPIase active site and formation of a ternary complex with the serine/threonine phosphatase, calcineurin, whose sequestration and inhibition prevents the dephosphorylation and activation of the nuclear factor for activation of T-cells (9 -12). Howe...

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.

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...

RPGR-interacting protein 1 (RPGRIP1) is a key component of cone and rod photoreceptor cells, where it interacts with RPGR (retinitis pigmentosa GTPase regulator). Mutations in RPGRIP1lead to autosomal recessive congenital blindness [Leber congenital amaurosis (LCA)]. Most LCA-associated missense mutations in RPGRIP1 are located in a segment that encodes two C2 domains. Based on the C2 domain of novel protein kinase C (PKC ), we built a 3D-homology model for the C-terminal C2 domain of RPGRIP1. This model revealed a potential Ca 2؉ -binding site that was predicted to be disrupted by a missense mutation in RPGRIP1, which was previously identified in an LCA patient. Through yeast two-hybrid screening of a retinal cDNA library, we found this C2 domain to specifically bind to nephrocystin-4, encoded by NPHP4. Mutations in NPHP4 are associated with nephronophthisis and a combination of nephronophthisis and retinitis pigmentosa called Senior-Løken syndrome (SLSN). We show that RPGRIP1 and nephrocystin-4 interact strongly in vitro and in vivo, and that they colocalize in the retina, matching the panretinal localization pattern of specific RPGRIP1 isoforms. Their interaction is disrupted by either mutations in RPGRIP1, found in patients with LCA, or by mutations in NPHP4, found in patients with nephronophthisis or SLSN. Thus, we provide evidence for the involvement of this disrupted interaction in the retinal dystrophy of both SLSN and LCA patients.T he X-linked gene RPGR (retinitis pigmentosa GTPase regulator) is mutated in patients with retinitis pigmentosa type 3 (RP3) (1, 2), cone or cone-rod dystrophy (COD1) (3, 4), atrophic macular degeneration (5), and RP in combination with impaired hearing and sinorespiratory infections (6). All RP3-associated missense mutations in RPGR have been identified in the N-terminal RCC1-homologous domain, and they disrupt the interaction with the C-terminal domain of RPGR-interacting protein 1 (RPGRIP1) (7,8). Mutations in RPGRIP1 lead to Leber congenital amaurosis (LCA), a genetically heterogeneous recessive disorder that is regarded to be the earliest and most severe form of all retinal dystrophies (9, 10). LCA accounts for at least 5% of all retinal dystrophies and is one of the main causes for blindness in children (11). RPGR and RPGRIP1 isoforms have been found to colocalize at the connecting cilia as well as the outer segments of rod and cone photoreceptors (12, 13). Differential localization among species has also been reported (12,14), and different isoforms of RPGRIP1 have been described resulting from splicing variation (7,14,15). The distinct partitioning of a subset of RPGRIP1 isoforms between the nuclear and cytoplasmic compartments combined with the differential and limited proteolysis of RPGRIP1 among retinal neurons, and the impact of LCA-linked mutations in RPGRIP1 in these processes, support the involvement of nucleocytoplasmatic signaling processes mediated by RPGRIP1 and its interacting partners in the pathogenesis of LCA, RP3, and allied diseases (15, 16). The pres...

The Ran-binding protein 2 (RanBP2) is a large mosaic protein with a pleiotropic role in cell function. Although the contribution of each partner and domain of RanBP2 to its biological functions are not understood, physiological deficits of RanBP2 downregulate glucose catabolism and energy homeostasis and lead to delocalization of mitochondria components in photosensory neurons. The kinesin-binding domain (KBD) of RanBP2 associates selectively in the central nervous system (CNS), and directly, with the ubiquitous and CNS-specific kinesins, KIF5B and KIF5C, respectively, but not with the highly homologous KIF5A. Here, we determine the molecular and biological bases of the selective interaction between RanBP2 and KIF5B/KIF5C. This interaction is conferred by a approximately 100-residue segment, comprising a portion of the coiled-coil and globular tail cargo-binding domains of KIF5B/KIF5C. A single residue conserved in KIF5B and KIF5C, but not KIF5A, confers KIF5-isotype-specific association with RanBP2. This interaction is also mediated by a conserved leucine-like heptad motif present in KIF5s and KBD of RanBP2. Selective inhibition of the interaction between KBD of RanBP2 and KIF5B/KIF5C in cell lines causes perinuclear clustering of mitochondria, but not of lysosomes, deficits in mitochondrial membrane potential and ultimately, cell shrinkage. Collectively, the data provide a rationale of the KIF5 subtype-specific interaction with RanBP2 and support a novel kinesin-dependent role of RanBP2 in mitochondria transport and function. The data also strengthen a model whereby the selection of a large array of cargoes for transport by a restricted number of motor proteins is mediated by adaptor proteins such as RanBP2.

The pathogenic drivers of sporadic and familial motor neuron disease (MND), such amyotrophic lateral sclerosis (ALS), are unknown. MND impairs the Ran GTPase cycle, which controls nucleocytoplasmic transport, ribostasis and proteostasis; however, cause-effect mechanisms of Ran GTPase modulators in motoneuron pathobiology have remained elusive. The cytosolic and peripheral nucleoporin Ranbp2 is a crucial regulator of the Ran GTPase cycle and of the proteostasis of neurological disease-prone substrates, but the roles of Ranbp2 in motoneuron biology and disease remain unknown. This study shows that conditional ablation of Ranbp2 in mouse Thy1 motoneurons causes ALS syndromes with hypoactivity followed by hindlimb paralysis, respiratory distress and, ultimately, death. These phenotypes are accompanied by: a decline in the nerve conduction velocity, free fatty acids and phophatidylcholine of the sciatic nerve; a reduction in the g-ratios of sciatic and phrenic nerves; and hypertrophy of motoneurons. Furthermore, Ranbp2 loss disrupts the nucleocytoplasmic partitioning of the import and export nuclear receptors importin β and exportin 1, respectively, Ran GTPase and histone deacetylase 4. Whole-transcriptome, proteomic and cellular analyses uncovered that the chemokine receptor Cxcr4, its antagonizing ligands Cxcl12 and Cxcl14, and effector, latent and activated Stat3 all undergo early autocrine and proteostatic deregulation, and intracellular sequestration and aggregation as a result of Ranbp2 loss in motoneurons. These effects were accompanied by paracrine and autocrine neuroglial deregulation of hnRNPH3 proteostasis in sciatic nerve and motoneurons, respectively, and post-transcriptional downregulation of metalloproteinase 28 in the sciatic nerve. Mechanistically, our results demonstrate that Ranbp2 controls nucleocytoplasmic, chemokine and metalloproteinase 28 signaling, and proteostasis of substrates that are crucial to motoneuronal homeostasis and whose impairments by loss of Ranbp2 drive ALS-like syndromes.

The function of the retinitis pigmentosa GTPase regulator interacting protein 1 (RPGRIP1) gene is currently not known. However, mutations within the gene lead to Leber Congenital Amaurosis and autosomal recessive retinitis pigmentosa in human patients. In a previously described knockout mouse model of the long splice variant of Rpgrip1, herein referred to as Rpgrip1(tm1Tili) mice, mislocalization of key outer segment proteins and dysmorphogenesis of outer segment discs preceded subsequent photoreceptor degeneration. In this report, we describe a new mouse model carrying a splice acceptor site mutation in Rpgrip1, herein referred to as Rpgrip1(nmf247) that is phenotypically distinct from Rpgrip1(tm1Tili) mice. Photoreceptor degeneration in homozygous Rpgrip1(nmf247) mice is earlier in onset and more severe when compared with Rpgrip1(tm1Tili) mice. Also, ultrastructural studies reveal that whereas Rpgrip1(nmf247) mutants have a normal structure and number of connecting cilia, unlike Rpgrip1(tm1Tili) mice, they do not elaborate rod outer segments (OS). Therefore, in addition to its role in OS disc morphogenesis, RPGRIP1 is essential for rod OS formation. Our study indicates the absence of multiple Rpgrip1 isoforms in Rpgrip1(nmf247) mice, suggesting different isoforms may play different roles in photoreceptors and underscores the importance of considering splice variants when generating targeted null mutations.