Proteostatic Remodeling of Small Heat Shock Chaperones─Crystallins by Ran-Binding Protein 2─and the Peptidyl-Prolyl 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.

Proteostatic remodeling of small heat shock chaperones - crystallins by Ran-binding protein 2 and the peptidyl-prolyl isomerase and chaperone activities of its cyclophilin domain.

Disturbances in phase transitions and intracellular partitions of nucleocytoplasmic shuttling substrates promote protein aggregation - a hallmark of neurodegenerative diseases. The modular Ran-binding protein 2 (Ranbp2) is a cytosolic molecular hub for rate-limiting steps of disassembly and phase transitions of Ran-GTP-bound protein ensembles exiting nuclear pores. Chaperones also play central roles in phase transitions and proteostasis by suppressing protein aggregation.

Microglial activation in an amyotrophic lateral sclerosis-like model caused by Ranbp2 loss and nucleocytoplasmic transport impairment in retinal ganglion neurons.

Nucleocytoplasmic transport is dysregulated in sporadic and familial amyotrophic lateral sclerosis (ALS) and retinal ganglion neurons (RGNs) are purportedly involved in ALS. The Ran-binding protein 2 (Ranbp2) controls rate-limiting steps of nucleocytoplasmic transport. Mice with Ranbp2 loss in Thy1-motoneurons develop cardinal ALS-like motor traits, but the impairments in RGNs and the degree of dysfunctional consonance between RGNs and motoneurons caused by Ranbp2 loss are unknown.

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.

Loss of Ranbp2 in motoneurons causes disruption of nucleocytoplasmic and chemokine signaling, proteostasis of hnRNPH3 and Mmp28, and development of amyotrophic lateral sclerosis-like syndromes.

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.

Uncoupling phototoxicity-elicited neural dysmorphology and death by insidious function and selective impairment of Ran-binding protein 2 (Ranbp2).

Morphological disintegration of neurons is coupled invariably to neural death. In particular, disruption of outer segments of photoreceptor neurons triggers photoreceptor death regardless of the pathological stressors. We show that Ranbp2(-/-)::Tg-Ranbp2(CLDm-HA) mice with mutations in SUMO-binding motif (SBM) of cyclophilin-like domain (CLD) of Ran-binding protein 2 (Ranbp2) expressed in a null Ranbp2 background lack untoward effects in photoreceptors in the absence of light-stress.

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.

Selective impairment of a subset of Ran-GTP-binding domains of ran-binding protein 2 (Ranbp2) suffices to recapitulate the degeneration of the retinal pigment epithelium (RPE) triggered by Ranbp2 ablation.

Retinal pigment epithelium (RPE) degeneration underpins diseases triggered by disparate genetic lesions, noxious insults, or both. The pleiotropic Ranbp2 controls the expression of intrinsic and extrinsic pathological stressors impinging on cellular viability. However, the physiological targets and mechanisms controlled by Ranbp2 in tissue homeostasis, such as RPE, are ill defined.

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.

Distinct and atypical intrinsic and extrinsic cell death pathways between photoreceptor cell types upon specific ablation of Ranbp2 in cone photoreceptors.

Non-autonomous cell-death is a cardinal feature of the disintegration of neural networks in neurodegenerative diseases, but the molecular bases of this process are poorly understood. The neural retina comprises a mosaic of rod and cone photoreceptors. Cone and rod photoreceptors degenerate upon rod-specific expression of heterogeneous mutations in functionally distinct genes, whereas cone-specific mutations are thought to cause only cone demise.

Kinesin-1 and mitochondrial motility control by discrimination of structurally equivalent but distinct subdomains in Ran-GTP-binding domains of Ran-binding protein 2.

The pleckstrin homology (PH) domain is a versatile fold that mediates a variety of protein-protein and protein-phosphatidylinositol lipid interactions. The Ran-binding protein 2 (RanBP2) contains four interspersed Ran GTPase-binding domains (RBD(n = 1-4)) with close structural homology to the PH domain of Bruton's tyrosine kinase. The RBD2, kinesin-binding domain (KBD) and RBD3 comprise a tripartite domain (R2KR3) of RanBP2 that causes the unfolding, microtubule binding and biphasic activation of kinesin-1, a crucial anterograde motor of mitochondrial motility.

Structural and functional plasticity of subcellular tethering, targeting and processing of RPGRIP1 by RPGR isoforms.

Mutations affecting the retinitis pigmentosa GTPase regulator-interacting protein 1 (RPGRIP1) interactome cause syndromic retinal dystrophies. RPGRIP1 interacts with the retinitis pigmentosa GTPase regulator (RPGR) through a domain homologous to RCC1 (RHD), a nucleotide exchange factor of Ran GTPase. However, functional relationships between RPGR and RPGRIP1 and their subcellular roles are lacking.

RANBP2 is an allosteric activator of the conventional kinesin-1 motor protein, KIF5B, in a minimal cell-free system.

The association of cargoes to kinesins is thought to promote kinesin activation, yet the validation of such a model with native cargoes is lacking because none is known to activate kinesins directly in an in vitro system of purified components. The RAN-binding protein 2 (RANBP2), through its kinesin-binding domain (KBD), associates in vivo with kinesin-1, KIF5B/KIF5C. Here, we show that KBD and its flanking domains, RAN GTPase-binding domains 2 and 3 (RBD2/RBD3), activate the ATPase activity of KIF5B approximately 30-fold in the presence of microtubules and ATP.

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.

The circling mouse (C57BL/6J-cir) has a 40-kilobase genomic deletion that includes the transmembrane inner ear (tmie) gene.

The circling mouse (C57BL6-cir) shows deafness and circling behavior in homozygotes. The mutation is transmitted with 100% penetrance by an autosomal recessive gene on chromosome 9. In the present study, we characterized the circling mutation as a 40-kilobase deletion that includes the transmembrane inner ear (tmie) gene.

Fine mapping of the circling (cir) gene on the distal portion of mouse chromosome 9.

Circling mice manifest profound deafness, head-tossing, and bi-directional circling behavior, which they inherit in autosomal recessive manner. Histologic examination of the inner ear reveals abnormalities of the region around the organ of Corti, spiral ganglion neurons, and outer hair cells. A genetic linkage map was constructed for an intraspecific backcross between cir and C57BL/6J mice.