Characterization of RanBP2-associated molecular components in neuroretina

Proteostatic Remodeling of Small Heat Shock Chaperones─Crystallins by Ran-Binding Protein 2─and the Peptidyl-Prolyl cis-trans Isomerase and Chaperone Activities of Its Cyclophilin Domain

Disturbances in protein phase transitions promote protein aggregation─a neurodegeneration hallmark. The modular Ran-binding protein 2 (Ranbp2) is a cytosolic molecular hub for rate-limiting steps of phase transitions of Ran-GTP-bound protein ensembles exiting nuclear pores. Chaperones also regulate phase transitions and proteostasis by suppressing protein aggregation. Ranbp2 haploinsufficiency promotes the age-dependent neuroprotection of the chorioretina against phototoxicity by proteostatic regulations of neuroprotective substrates of Ranbp2 and by suppressing the buildup of polyubiquitylated substrates. Losses of peptidyl-prolyl cis-trans isomerase (PPIase) and chaperone activities of the cyclophilin domain (CY) of Ranbp2 recapitulate molecular effects of Ranbp2 haploinsufficiency. These CY impairments also stimulate deubiquitylation activities and phase transitions of 19S cap subunits of the 26S proteasome that associates with Ranbp2. However, links between CY moonlighting activity, substrate ubiquitylation, and proteostasis remain incomplete. Here, we reveal the Ranbp2 regulation of small heat shock chaperones─crystallins in the chorioretina by proteomics of mice with total or selective modular deficits of Ranbp2. Specifically, loss of CY PPIase of Ranbp2 upregulates αA-Crystallin, which is repressed in adult nonlenticular tissues. Conversely, impairment of CY's chaperone activity opposite to the PPIase pocket downregulates a subset of αA-Crystallin's substrates, γ-crystallins. These CY-dependent effects cause age-dependent and chorioretinal-selective declines of ubiquitylated substrates without affecting the chorioretinal morphology. A model emerges whereby inhibition of Ranbp2's CY PPIase remodels crystallins' expressions, subdues molecular aging, and preordains the chorioretina to neuroprotection by augmenting the chaperone capacity and the degradation of polyubiquitylated substrates against proteostatic impairments. Further, the druggable Ranbp2 CY holds pan-therapeutic potential against proteotoxicity and neurodegeneration.

Nucleocytoplasmic transport at the crossroads of proteostasis, neurodegeneration and neuroprotection

Nucleocytoplasmic transport comprises the multistep assembly, transport, and disassembly of protein and RNA cargoes entering and exiting nuclear pores. Accruing evidence supports that impairments to nucleocytoplasmic transport are a hallmark of neurodegenerative diseases. These impairments cause dysregulations in nucleocytoplasmic partitioning and proteostasis of nuclear transport receptors and client substrates that promote intracellular deposits - another hallmark of neurodegeneration. Disturbances in liquid-liquid phase separation (LLPS) between dense and dilute phases of biomolecules implicated in nucleocytoplasmic transport promote micrometer-scale coacervates, leading to proteinaceous aggregates. This Review provides historical and emerging principles of LLPS at the interface of nucleocytoplasmic transport, proteostasis, aging and noxious insults, whose dysregulations promote intracellular aggregates. E3 SUMO-protein ligase Ranbp2 constitutes the cytoplasmic filaments of nuclear pores, where it acts as a molecular hub for rate-limiting steps of nucleocytoplasmic transport. A vignette is provided on the roles of Ranbp2 in nucleocytoplasmic transport and at the intersection of proteostasis in the survival of photoreceptor and motor neurons under homeostatic and pathophysiological environments. Current unmet clinical needs are highlighted, including therapeutics aiming to manipulate aggregation-dissolution models of purported neurotoxicity in neurodegeneration.

Impairments in age-dependent ubiquitin proteostasis and structural integrity of selective neurons by uncoupling Ran GTPase from the Ran-binding domain 3 of Ranbp2 and identification of novel mitochondrial isoforms of ubiquitin-conjugating enzyme E2I (ubc9) and Ranbp2

The Ran-binding protein 2 (Ranbp2/Nup358) is a cytoplasmic and peripheral nucleoporin comprised of 4 Ran-GTP-binding domains (RBDs) that are interspersed among diverse structural domains with multifunctional activities. Our prior studies found that the RBD2 and RBD3 of Ranbp2 control mitochondrial motility independently of Ran-GTP-binding in cultured cells, whereas loss of Ran-GTP-binding to RBD2 and RBD3 are essential to support cone photoreceptor development and the survival of mature retinal pigment epithelium (RPE) in mice. Here, we uncover that loss of Ran-GTP-binding to RBD3 alone promotes the robust age-dependent increase of ubiquitylated substrates and S1 subunit (Pmsd1) of the 19S cap of the proteasome in the retina and RPE and that such loss in RBD3 also compromises the structural integrity of the outer segment compartment of cone photoreceptors only and without affecting the viability of these neurons. We also found that the E2-ligase and partner of Ranbp2, ubc9, is localized prominently in the mitochondrial-rich ellipsoid compartment of photoreceptors, where Ranbp2 is also known to localize with and modulate the activity of mitochondrial proteins. However, the natures of Ranbp2 and ubc9 isoforms to the mitochondria are heretofore elusive. Subcellular fractionation, co-immunolocalization and immunoaffinity purification of Ranbp2 complexes show that novel isoforms of Ranbp2 and ubc9 with molecular masses distinct from the large Ranbp2 and unmodified ubc9 isoforms localize specifically to the mitochondrial fraction or associate with mitochondrial components, whereas unmodified and SUMOylated Ran GTPase are excluded from the mitochondrial fraction. Further, liposome-mediated intracellular delivery of an antibody against a domain shared by the mitochondrial and nuclear pore isoforms of Ranbp2 causes the profound fragmentation of mitochondria and their delocalization from Ranbp2 and without affecting Ranbp2 localization at the nuclear pores. Collectively, the data support that Ran GTPase-dependent and independent and moonlighting roles of Ranbp2 or domains thereof and ubc9 control selectively age-dependent, neural-type and mitochondrial functions.

Targeting the cyclophilin domain of Ran-binding protein 2 (Ranbp2) with novel small molecules to control the proteostasis of STAT3, hnRNPA2B1 and M-opsin

Cyclophilins are peptidyl cis-trans prolyl isomerases (PPIases), whose activity is typically inhibited by cyclosporine A (CsA), a potent immunosuppressor. Cyclophilins are also chaperones. Emerging evidence supports that cyclophilins present nonoverlapping PPIase and chaperone activities. The proteostasis of the disease-relevant substrates, signal transducer and activator of transcription 3 and 5 (STAT3/STAT5), heterogeneous nuclear ribonucleoprotein A2B1 (hnRNPA2B1), and M-opsin, is regulated by nonoverlapping chaperone and PPIase activities of the cyclophilin domain (CY) of Ranbp2, a multifunctional and modular scaffold that controls nucleocytoplasmic shuttling and proteostasis of selective substrates. Although highly homologous, CY and the archetypal cyclophilin A (CyPA) present distinct catalytic and CsA-binding activities owing to unique structural features between these cylophilins. We explored structural idiosyncrasies between CY and CyPA to screen in silico nearly 9 million small molecules (SM) against the CY PPIase pocket and identify SMs with selective bioactivity toward STAT3, hnRNPA2B1, or M-opsin proteostasis. We found three classes of SMs that enhance the cytokine-stimulated transcriptional activity of STAT3 without changing latent and activated STAT3 levels, down-regulate hnRNPA2B1 or M-opsin proteostasis, or a combination of these. Further, a SM that suppresses hnRNPA2B1 proteostasis also inhibits strongly and selectively the PPIase activity of CY. This study unravels chemical probes for multimodal regulation of CY of Ranbp2 and its substrates, and this regulation likely results in the allosterism stemming from the interconversion of conformational substates of cyclophilins. The results also demonstrate the feasibility of CY in drug discovery against disease-relevant substrates controlled by Ranbp2, and they open new opportunities for therapeutic interventions.

Differential loss of prolyl isomerase or chaperone activity of Ran-binding protein 2 (Ranbp2) unveils distinct physiological roles of its cyclophilin domain in proteostasis

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. Stress-induced STAT3 activation is also unaffected in Tg-Ranbp2(R2944A-HA)::Ranbp2(-/-). Conversely, proteomic analyses found that the multisystem proteinopathy/amyotrophic lateral sclerosis proteins, heterogeneous nuclear ribonucleoproteins A2/B1, are down-regulated post-transcriptionally only in Tg-Ranbp2(R2944A-HA)::Ranbp2(-/-). This is accompanied by the age- and tissue-dependent reductions of diubiquitin and ubiquitylated proteins, increased deubiquitylation activity, and accumulation of the 26 S proteasome subunits S1 and S5b. These manifestations are absent in another line, Tg-Ranbp2(CLDm-HA)::Ranbp2(-/-), harboring SUMO-1 and S1-binding mutations in the Ranbp2 cyclophilin-like domain. These results unveil distinct mechanistic and biological links between PPIase and chaperone activities of Ranbp2 cyclophilin toward proteostasis of selective substrates and with novel therapeutic potential.

In vitro interaction of tubulin with the photoreceptor cGMP phosphodiesterase gamma-subunit

The alpha and beta tubulins compose the microtubule cytoskeleton which is involved in many cellular processes such as vesicular transport. The photoreceptor cells in the retina are neurons specialized for phototransduction. Here we report a novel interaction between tubulin and the photoreceptor cGMP phosphodiesterase (PDE6) gamma subunit (PDE gamma). The specificity and molecular details of the PDE gamma:tubulin interaction were analyzed through the experiments of pull down, microtubule co-sedimentation, and NMR spectroscopy. The tubulin-interacting site was identified to be in the PDE gamma C-terminal I67-G85 region, and the interaction interface appeared to be distinct from those with the other PDE gamma targets in phototransduction. We also observed that PDE gamma interacted with tubulin in a GTP-dependent manner. Our findings offer implications for non-phototransduction role(s) of PDE gamma in the photoreceptor neurons.

Neuroprotection resulting from insufficiency of RANBP2 is associated with the modulation of protein and lipid homeostasis of functionally diverse but linked pathways in response to oxidative stress

Oxidative stress is a deleterious stressor associated with a plethora of disease and aging manifestations, including neurodegenerative disorders, yet very few factors and mechanisms promoting the neuroprotection of photoreceptor and other neurons against oxidative stress are known. Insufficiency of RAN-binding protein-2 (RANBP2), a large, mosaic protein with pleiotropic functions, suppresses apoptosis of photoreceptor neurons upon aging and light-elicited oxidative stress, and promotes age-dependent tumorigenesis by mechanisms that are not well understood. Here we show that, by downregulating selective partners of RANBP2, such as RAN GTPase, UBC9 and ErbB-2 (HER2; Neu), and blunting the upregulation of a set of orphan nuclear receptors and the light-dependent accumulation of ubiquitylated substrates, light-elicited oxidative stress and Ranbp2 haploinsufficiency have a selective effect on protein homeostasis in the retina. Among the nuclear orphan receptors affected by insufficiency of RANBP2, we identified an isoform of COUP-TFI (Nr2f1) as the only receptor stably co-associating in vivo with RANBP2 and distinct isoforms of UBC9. Strikingly, most changes in proteostasis caused by insufficiency of RANBP2 in the retina are not observed in the supporting tissue, the retinal pigment epithelium (RPE). Instead, insufficiency of RANBP2 in the RPE prominently suppresses the light-dependent accumulation of lipophilic deposits, and it has divergent effects on the accumulation of free cholesterol and free fatty acids despite the genotype-independent increase of light-elicited oxidative stress in this tissue. Thus, the data indicate that insufficiency of RANBP2 results in the cell-type-dependent downregulation of protein and lipid homeostasis, acting on functionally interconnected pathways in response to oxidative stress. These results provide a rationale for the neuroprotection from light damage of photosensory neurons by RANBP2 insufficiency and for the identification of novel therapeutic targets and approaches promoting neuroprotection.

RPGRIP1 is essential for normal rod photoreceptor outer segment elaboration and morphogenesis

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.

Gene and protein expression pilot profiling and biomarkers in an experimental mouse model of hypertensive glaucoma

Glaucoma is a group of genetically heterogeneous neurodegenerative disorders causing the degeneration of the ganglion neurons of the retina. Increased intraocular pressure (IOP) is a hallmark risk factor promoting the death of ganglion neurons of the retina in glaucoma. Yet, the molecular processes underlying the degeneration of these neurons by increased IOP are not understood. To gain insight into the early molecular events and discover biomarkers induced by IOP, we performed gene and protein expression profiling to compare retinas of eyes with and without high IOP in a rodent model of experimental glaucoma. This pilot study found that the IOP-mediated changes in the transcription levels of a restricted set of genes implicated in peroxisomal and mitochondrial function, modulation of neuron survival and inflammatory processes, were also accompanied by changes in the levels of proteins encoded by the same genes. With the exception of the inflammatory markers, serum amyloid-A1 (SAA1) and serum amyloid-A2 (SAA2), the IOP-induced changes in protein expression were restricted to ganglion neurons of the retina and they were detected also in the vitreous, thus suggesting an early IOP-mediated loss of ganglion cell integrity. Interestingly, SAA1 and SAA2 were induced in retinal microglia cells, whereas they were reduced in sera of IOP-responsive mice. Hence, this study defines novel IOP-induced molecular processes, biomarkers and sources thereof, and it further validates the extension of the analyses herein reported to other genes modulated by IOP.

Haploinsufficiency of RanBP2 is neuroprotective against light-elicited and age-dependent degeneration of photoreceptor neurons

Prolonged light exposure is a determinant factor in inducing neurodegeneration of photoreceptors by apoptosis. Yet, the molecular bases of the pathways and components triggering this cell death event are elusive. Here, we reveal a prominent age-dependent increase in the susceptibility of photoreceptor neurons to undergo apoptosis under light in a mouse model. This is accompanied by light-induced subcellular changes of photoreceptors, such as dilation of the disks at the tip of the outer segments, prominent vesiculation of nascent disks, and autophagy of mitochondria into large multilamellar bodies. Notably, haploinsufficiency of Ran-binding protein-2 (RanBP2) suppresses apoptosis and most facets of membrane dysgenesis observed with age upon light-elicited stress. RanBP2 haploinsufficiency promotes decreased levels of free fatty acids in the retina independent of light exposure and turns the mice refractory to weight gain on a high-fat diet, whereas light promotes an increase in hydrogen peroxide regardless of the genotype. These studies demonstrate the presence of age-dependent and RanBP2-mediated pathways modulating membrane biogenesis of the outer segments and light-elicited neurodegeneration of photoreceptors. Furthermore, the findings support a mechanism whereby the RanBP2-dependent production of free fatty acids, metabolites thereof or the modulation of a cofactor dependent on any of these, promote apoptosis of photoreceptors in concert with the light-stimulated production of reactive oxygen species.

The cyclophilin-like domain of Ran-binding protein-2 modulates selectively the activity of the ubiquitin-proteasome system and protein biogenesis

The ubiquitin-proteasome system (UPS) plays a critical role in protein degradation. The 19S regulatory particle (RP) of the 26S proteasome mediates the recognition, deubiquitylation, unfolding, and channeling of ubiquitylated substrates to the 20S proteasome. Several subunits of the 19S RP interact with a growing number of factors. The cyclophilin-like domain (CLD) of Ran-binding protein-2 (RanBP2/Nup358) associates specifically with at least one subunit, S1, of the base subcomplex of the 19S RP, but the functional implications of this interaction on the UPS activity are elusive. This study shows the CLD of RanBP2 promotes selectively the accumulation of a subset of reporter substrates of the UPS, such as the ubiquitin (Ub)-fusion yellow fluorescent protein (YFP) degradation substrate, Ub(G76V)-YFP, and the N-end rule substrate, Ub-R-YFP. Conversely, the degradation of endoplasmic reticulum and misfolded proteins, and of those linked to UPS-independent degradation, is not affected by CLD. The selective effect of CLD on the UPS in vivo is independent of, and synergistic with, proteasome inhibitors, and CLD does not affect the intrinsic proteolytic activity of the 20S proteasome. The inhibitory activity of CLD on the UPS resides in a purported SUMO binding motif. We also found two RanBP2 substrates, RanGTPase-activating protein and retinitis pigmentosa GTPase regulator interacting protein-1alpha1, whose steady-state levels are selectively modulated by CLD. Hence, the CLD of RanBP2 acts as a novel auxiliary modulator of the UPS activity; it may contribute to the molecular and subcellular compartmentation of the turnover of properly folded proteins and modulation of the expressivity of several neurological diseases.

Association of the Kinesin‐Binding Domain of RanBP2 to KIF5B and KIF5C Determines Mitochondria Localization and Function

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.

RanBP2 modulates Cox11 and hexokinase I activities and haploinsufficiency of RanBP2 causes deficits in glucose metabolism

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 large protein with several domains. Although several protein partners were found to interact with selective domains of RanBP2, none to this date were found toward its large leucine-rich domain (LD). Cell-based experiments support several roles of RanBP2 in cell function, such as the production of functional proteins, control of protein trafficking between the nuclear and cytosol compartments, and control of multiple facets underlying cell division. Still, the genetic and physiological implications of the interactions between RanBP2 and its partners and of the function of RanBP2 in a whole-animal model remain elusive. The authors report the identification of two novel mitochondrial partners of the LD of RanBP2, Cox11 and hexokinase type I (HKI); and with multidisciplinary approaches probe the role of RanBP2 and its LD on Cox11, HKI, and functions allied to these. The authors found that RanBP2 exhibits chaperone activity toward HKI and Cox11. RanBP2 and Cox11 profoundly modulate HKI activity. Moreover, partial loss-of-function of RanBP2 in a mouse model induces deficits in growth rates and breakdown of glucose, promotes the down-regulation of HKI and ATP levels selectively in the central nervous system, and impairs visual function. These findings support a critical role of RanBP2 and its partners in metabolic processes and allied disease states.

Limited proteolysis differentially modulates the stability and subcellular localization of domains of RPGRIP1 that are distinctly affected by mutations in Leber's congenital amaurosis

The retinitis pigmentosa GTPase regulator (RPGR) protein interacts with the retinitis pigmentosa GTPase regulator interacting protein-1 (RPGRIP1). Genetic lesions in the cognate genes lead to distinct and severe human retinal dystrophies. The biological role of these proteins in retinal function and pathogenesis of retinal diseases is elusive. Here, we present the first physiological assay of the role of RPGRIP1 and mutations therein. We found that the monoallelic and homozygous mutations, DeltaE1279 and D1114G, in the RPGR-interacting domain (RID) of RPGRIP1, enhance and abolish, respectively, its interaction in vivo with RPGR without affecting the stability of RID. In contrast to RID(WT) and RID(D1114G), chemical genetics shows that the interaction of RID(DeltaE1279) with RPGR is resistant to various stress treatments such as osmotic, pH and heat-shock stimuli. Hence, RID(D1114G) and RID(DeltaE1279) constitute loss- and gain-of-function mutations. Moreover, we find that the isoforms, bRPGRIP1 and bRPGRIP1b, undergo limited proteolysis constitutively in vivo in the cytoplasm compartment. This leads to the relocation and accumulation of a small and stable N-terminal domain of approximately 7 kDa to the nucleus, whereas the cytosolic C-terminal domain of RPGRIP1 is degraded and short-lived. The RID(D1114G) and RID(DeltaE1279) mutations exhibit strong cis-acting and antagonistic biological effects on the nuclear relocation, subcellular distribution and proteolytic cleavage of RPGRIP1 and/or domains thereof. These data support distinct and spatiotemporal subcellular-specific roles to RPGRIP1. A novel RPGRIP1-mediated nucleocytoplasmic crosstalk and transport pathway regulated by RID, and hence by RPGR, emerges with implications in the molecular pathogenesis of retinopathies, and a model to other diseases.

Identification of RanBP2- and kinesin-mediated transport pathways with restricted neuronal and subcellular localization

Ran-binding proteins, karyopherins, and RanGTPase mediate and impart directionality to nucleocytoplasmic transport processes. This biological process remains elusive in neurons. RanBP2 has been localized at the nuclear pore complexes and is very abundant in the neuroretina. RanBP2 mediates the assembly of a large complex comprising RanGTPase, CRM1/exportin-1, importin-beta, KIF5-motor proteins, components of the 19S cap of the 26S proteasome, ubc9 and opsin. Here, we show RanBP2 is abundant in the ellipsoid compartment of photoreceptors and RanGTPase-positive particles in cytoplasmic tracks extending away from the nuclear envelope of subpopulations of ganglion cells, suggesting RanBP2's release from nuclear pore complexes. KIF5C and KIF5B are specifically expressed in a subset of neuroretinal cells and differentially localize with RanBP2 and importin-beta in distinct compartments. The C-terminal domains of KIF5B and KIF5C, but not KIF5A, associate directly with importin-beta in a RanGTPase-dependent fashion in vivo and in vitro, indicating importin-beta is an endogenous cargo for a subset of KIF5s in retinal neurons. The KIF5 transport pathway is absent from the myoid region of a topographically distinct subclass of blue cones and the distribution of kinesin-light chains is largely distinct from its KIF5 partners. Altogether, the results identify the existence of neuronal- and subtype-specific kinesin-mediated transport pathways of importin-beta-bound cargoes to and/or from RanBP2 and indicate RanBP2 itself may also constitute a scaffold carrier for some of its associated partners. The implications of these findings in protein kinesis and pathogenesis of degenerative neuropathies are discussed.

The docking of kinesins, KIF5B and KIF5C, to Ran-binding protein 2 (RanBP2) is mediated via a novel RanBP2 domain

The Ran-binding protein 2 (RanBP2) is a vertebrate mosaic protein composed of four interspersed RanGTPase binding domains (RBDs), a variable and species-specific 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 microtubule-based 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 zinc finger cluster domain of RanBP2 is a specific docking site for the nuclear export factor, exportin-1

The Ran-binding protein 2 (RanBP2) is a large scaffold cyclophilin-related protein expressed in photoreceptor cells. Red/green opsin, Ran-GTPase, and the 19 S regulatory complex of the proteasome associate with specific RanBP2 structural modules. Some of these play a role in chaperoning the functional expression of opsin. RanBP2 localization at cytoplasmic fibrils emanating from the nuclear pore complex and interaction with the Ran-GTPase support also its role in nucleocytoplasmic transport processes. The degenerate nucleoporin repeat motifs FXFG, GLFG, and XXFG have been proposed to mediate the movement of nucleocytoplasmic transport factors. In particular, RanBP2 has been implicated in nuclear import processes. Here, we show the zinc fingers of RanBP2 associate with high specificity to the nuclear export factor, exportin-1 (CRM1). The bovine RanBP2 transcript contained only five of the eight zinc fingers reported in the human counterpart and are sufficient for exportin-1 association with RanBP2. In contrast to Ran interaction with RanBP2-exportin-1 complex, exportin-1 binding to the zinc finger cluster domain of RanBP2 is insensitive to leptomycin B and nucleotide-bound state of Ran-GTPase. Our results indicate that the zinc finger-rich domain of RanBP2 constitutes a docking site for exportin-1 during nuclear export. Thus, RanBP2 emerges as a key component of the nuclear export pathway.