Interconversion of red opsin isoforms by the cyclophilin-related chaperone protein Ran-binding protein 2

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.

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 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. Ranbp2 haploinsufficiency promotes the age-dependent neuroprotection of the chorioretina against photo-oxidative stress by proteostatic regulations of Ranbp2 substrates and by countering the build-up of poly-ubiquitylated substrates. Further, the peptidyl-prolyl cis-trans isomerase (PPIase) and chaperone activities of the cyclophilin domain (CY) of Ranbp2 modulate the proteostasis of selective neuroprotective substrates, such as hnRNPA2B1, STAT3, HDAC4 or L/M-opsin, while promoting a decline of ubiquitylated substrates. However, links between CY PPIase activity on client substrates and its effect(s) on ubiquitylated substrates are unclear. Here, proteomics of genetically modified mice with deficits of Ranbp2 uncovered the regulation of the small heat shock chaperones - crystallins by Ranbp2 in the chorioretina. Loss of CY PPIase of Ranbp2 up-regulates αA-crystallin proteostasis, which is repressed in non-lenticular tissues. Conversely, the αA-crystallin's substrates, γ-crystallins, are down-regulated by impairment of CY's C-terminal chaperone activity. These CY-dependent effects cause the age-dependent decline of ubiquitylated substrates without overt chorioretinal morphological changes. A model emerges whereby the Ranbp2 CY-dependent remodeling of crystallins' proteostasis subdues molecular aging and preordains chorioretinal neuroprotection by augmenting the chaperone buffering capacity and the decline of ubiquitylated substrates against proteostatic impairments. Further, CY's moonlighting activity holds pan -therapeutic potential against neurodegeneration.

RanBP2/Nup358 Mediates Sumoylation of STAT1 and Antagonizes Interferon-α-Mediated Antiviral Innate Immunity

Type I interferon (IFN-I)-induced signaling plays a critical role in host antiviral innate immune responses. Despite this, the mechanisms that regulate this signaling pathway have yet to be fully elucidated. The nucleoporin Ran Binding Protein 2 (RanBP2) (also known as Nucleoporin 358 KDa, Nup358) has been implicated in a number of cellular processes, including host innate immune signaling pathways, and is known to influence viral infection. In this study, we documented that RanBP2 mediates the sumoylation of signal transducers and activators of transcription 1 (STAT1) and inhibits IFN-α-induced signaling. Specifically, we found that RanBP2-mediated sumoylation inhibits the interaction of STAT1 and Janus kinase 1 (JAK1), as well as the phosphorylation and nuclear accumulation of STAT1 after IFN-α stimulation, thereby antagonizing the IFN-α-mediated antiviral innate immune signaling pathway and promoting viral infection. Our findings not only provide insights into a novel function of RanBP2 in antiviral innate immunity but may also contribute to the development of new antiviral therapeutic strategies.

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.

Roles of Nucleoporin RanBP2/Nup358 in Acute Necrotizing Encephalopathy Type 1 (ANE1) and Viral Infection

Ran Binding Protein 2 (RanBP2 or Nucleoporin358) is one of the main components of the cytoplasmic filaments of the nuclear pore complex. Mutations in the RANBP2 gene are associated with acute necrotizing encephalopathy type 1 (ANE1), a rare condition where patients experience a sharp rise in cytokine production in response to viral infection and undergo hyperinflammation, seizures, coma, and a high rate of mortality. Despite this, it remains unclear howRanBP2 and its ANE1-associated mutations contribute to pathology. Mounting evidence has shown that RanBP2 interacts with distinct viruses to regulate viral infection. In addition, RanBP2 may regulate innate immune response pathways. This review summarizes recent advances in our understanding of how mutations in RANBP2 contribute to ANE1 and discusses how RanBP2 interacts with distinct viruses and affects viral infection. Recent findings indicate that RanBP2 might be an important therapeutic target, not only in the suppression of ANE1-driven cytokine storms, but also to combat hyperinflammation in response to viral infections.

Sodium Iodate-Induced Mouse Model of Age-Related Macular Degeneration Displayed Altered Expression Patterns of Sumoylation Enzymes E1, E2 and E3

PURPOSE

Protein sumoylation is a highly dynamic and reversible post-translational modification, involving covalently conjugation of the small ubiquitin-like modifier (SUMO) to the lysine residue of the target protein. Similar to ubiquitination, sumoylation is catalyzed by E1, E2 and several E3 ligases. However, sumoylation usually does not cause protein degradation but alter the target function through diverse mechanisms. Increasing evidences have shown that sumoylation plays pivotal roles in the pathogenesis of human diseases, including neuron degeneration, cancer and heart disease, etc. We and others have shown that sumoylation is critically implicated in mouse eye development. However, the expression of sumoylation machinery has not been characterized in normal and pathogenic retina. Worldwide, age-related macular degeneration (AMD) is the leading cause of irreversible blindness in aged person. In the present study, we investigated the expression of the major sumoylation enzymes in normal mice and sodium iodateinduced AMD mouse model.

METHODS

Four-week-old C57BL/6J mice were used in our experiment. A sterile 1% NaIO3 solution was freshly prepared in PBS from solid NaIO3. Experimental mice were injected with 70 mg/kg NaIO3, and similar volumes of PBS as control. Eyes were enucleated and immersion in FAA fixation overnight and processed for eye cross-sections. After fixation, cross sections eyes were dehydrated, embedded in paraffin, and 6 mm transverse sections were cut using the rotary microtome. Then paraffin sections were stained with hematoxylin and eosin (H&E), and mouse retinal thickness was observed to assess the histopathologic changes.

RESULTS

Significantly declined RNA levels of E1, E2 and E3 ligase PIAS1 in NaIO3-injected mouse RPE one day-post treatment. Consistently, the protein level of PIAS1 was also decreased at this time point. At the late stage of treatment (three days post-injection), significantly reduced expression of E1 enzyme SAE1/UBA2 was detected in NaIO3-injected mouse retinas. In the contrary, dramatically increased E3 ligase RanBP2 was found in the injected-retinas.

CONCLUSION

Together, our results demonstrated for the first time the dynamic expression of sumoylation pathway enzymes during the progression of retina degeneration induced by oxidative stress. Dynamic expression of E1, E2 and E3 enzymes were found during the time course of RPE and retina degeneration, which revealed the potential regulatory roles of sumoylation in AMD pathogenesis.

The coming-of-age of nucleocytoplasmic transport in motor neuron disease and neurodegeneration

The nuclear pore is the gatekeeper of nucleocytoplasmic transport and signaling through which a vast flux of information is continuously exchanged between the nuclear and cytoplasmic compartments to maintain cellular homeostasis. A unifying and organizing principle has recently emerged that cements the notion that several forms of amyotrophic lateral sclerosis (ALS), and growing number of other neurodegenerative diseases, co-opt the dysregulation of nucleocytoplasmic transport and that this impairment is a pathogenic driver of neurodegeneration. The understanding of shared pathomechanisms that underpin neurodegenerative diseases with impairments in nucleocytoplasmic transport and how these interface with current concepts of nucleocytoplasmic transport is bound to illuminate this fundamental biological process in a yet more physiological context. Here, I summarize unresolved questions and evidence and extend basic and critical concepts and challenges of nucleocytoplasmic transport and its role in the pathogenesis of neurodegenerative diseases, such as ALS. These principles will help to appreciate the roles of nucleocytoplasmic transport in the pathogenesis of ALS and other neurodegenerative diseases, and generate a framework for new ideas of the susceptibility of motoneurons, and possibly other neurons, to degeneration by dysregulation of nucleocytoplasmic transport.

The genomic basis of cichlid fish adaptation within the deepwater "twilight zone" of Lake Malawi

Deepwater environments are characterized by low levels of available light at narrow spectra, great hydrostatic pressure, and low levels of dissolved oxygen—conditions predicted to exert highly specific selection pressures. In Lake Malawi over 800 cichlid species have evolved, and this adaptive radiation extends into the “twilight zone” below 50 m. We use population‐level RAD‐seq data to investigate whether four endemic deepwater species (Diplotaxodon spp.) have experienced divergent selection within this environment. We identify candidate genes including regulators of photoreceptor function, photopigments, lens morphology, and haemoglobin, many not previously implicated in cichlid adaptive radiations. Colocalization of functionally linked genes suggests coadapted “supergene” complexes. Comparisons of Diplotaxodon to the broader Lake Malawi radiation using genome resequencing data revealed functional substitutions and signatures of positive selection in candidate genes. Our data provide unique insights into genomic adaptation within deepwater habitats, and suggest genome‐level specialization for life at depth as an important process in cichlid radiation.

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. 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.

Summary: Loss of Ranbp2 in spinal motoneurons drives ALS syndromes in mice and Ranbp2 functions in nucleocytoplasmic trafficking, proteostasis and chemokine signaling uncover novel therapeutic targets and mechanisms for motoneuron disease.

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.

Chaperoning G protein-coupled receptors: from cell biology to therapeutics

G protein-coupled receptors (GPCRs) are membrane proteins that traverse the plasma membrane seven times (hence, are also called 7TM receptors). The polytopic structure of GPCRs makes the folding of GPCRs difficult and complex. Indeed, many wild-type GPCRs are not folded optimally, and defects in folding are the most common cause of genetic diseases due to GPCR mutations. Both general and receptor-specific molecular chaperones aid the folding of GPCRs. Chemical chaperones have been shown to be able to correct the misfolding in mutant GPCRs, proving to be important tools for studying the structure-function relationship of GPCRs. However, their potential therapeutic value is very limited. Pharmacological chaperones (pharmacoperones) are potentially important novel therapeutics for treating genetic diseases caused by mutations in GPCR genes that resulted in misfolded mutant proteins. Pharmacoperones also increase cell surface expression of wild-type GPCRs; therefore, they could be used to treat diseases that do not harbor mutations in GPCRs. Recent studies have shown that indeed pharmacoperones work in both experimental animals and patients. High-throughput assays have been developed to identify new pharmacoperones that could be used as therapeutics for a number of endocrine and other genetic diseases.

Nucleoporin Gene Fusions and Hematopoietic Malignancies

Nuclear pore complexes (NPCs) are the sole gateways between the nucleus and the cytoplasm of eukaryotic cells and they mediate all macromolecular trafficking between these cellular compartments. Nucleocytoplasmic transport is highly selective and precisely regulated and as such an important aspect of normal cellular function. Defects in this process or in its machinery have been linked to various human diseases, including cancer. Nucleoporins, which are about 30 proteins that built up NPCs, are critical players in nucleocytoplasmic transport and have also been shown to be key players in numerous other cellular processes, such as cell cycle control and gene expression regulation. This review will focus on the three nucleoporins Nup98, Nup214, and Nup358. Common to them is their significance in nucleocytoplasmic transport, their multiple other functions, and being targets for chromosomal translocations that lead to haematopoietic malignancies, in particular acute myeloid leukaemia. The underlying molecular mechanisms of nucleoporin-associated leukaemias are only poorly understood but share some characteristics and are distinguished by their poor prognosis and therapy outcome.

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.

Viral and cellular requirements for the nuclear entry of retroviral preintegration nucleoprotein complexes

Retroviruses integrate their reverse transcribed genomes into host cell chromosomes as an obligate step in virus replication. The nuclear envelope separates the chromosomes from the cell cytoplasm during interphase, and different retroviral groups deal with this physical barrier in different ways. Gammaretroviruses are dependent on the passage of target cells through mitosis, where they are believed to access chromosomes when the nuclear envelope dissolves for cell division. Contrastingly, lentiviruses such as HIV-1 infect non-dividing cells, and are believed to enter the nucleus by passing through the nuclear pore complex. While numerous virally encoded elements have been proposed to be involved in HIV-1 nuclear import, recent evidence has highlighted the importance of HIV-1 capsid. Furthermore, capsid was found to be responsible for the viral requirement of various nuclear transport proteins, including transportin 3 and nucleoporins NUP153 and NUP358, during infection. In this review, we describe our current understanding of retroviral nuclear import, with emphasis on recent developments on the role of the HIV-1 capsid protein.

HIV-1 capsid undergoes coupled binding and isomerization by the nuclear pore protein NUP358

Background

Lentiviruses such as HIV-1 can be distinguished from other retroviruses by the cyclophilin A-binding loop in their capsid and their ability to infect non-dividing cells. Infection of non-dividing cells requires transport through the nuclear pore but how this is mediated is unknown.

Results

Here we present the crystal structure of the N-terminal capsid domain of HIV-1 in complex with the cyclophilin domain of nuclear pore protein NUP358. The structure reveals that HIV-1 is positioned to allow single-bond resonance stabilisation of exposed capsid residue P90. NMR exchange experiments demonstrate that NUP358 is an active isomerase, which efficiently catalyzes cis-trans isomerization of the HIV-1 capsid. In contrast, the distantly related feline lentivirus FIV can bind NUP358 but is neither isomerized by it nor requires it for infection.

Conclusion

Isomerization by NUP358 may be preserved by HIV-1 to target the nuclear pore and synchronize nuclear entry with capsid uncoating.

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. However, the interplay between Ran GTPase and R₂KR₃ of RanBP2 in kinesin-1 activation and mitochondrial motility is elusive. We use structure–function, biochemical, kinetic and cell-based assays with time-lapse live-cell microscopy of over 260 000 mitochondrial-motility-related events to find mutually exclusive subdomains in RBD₂ and RBD₃ towards Ran GTPase binding, kinesin-1 activation and mitochondrial motility regulation. The RBD₂ and RBD₃ exhibit Ran-GTP-independent, subdomain and stereochemical-dependent discrimination on the biphasic kinetics of kinesin-1 activation or regulation of mitochondrial motility. Further, KBD alone and R₂KR₃ stimulate and suppress, respectively, multiple biophysical parameters of mitochondrial motility. The regulation of the bidirectional transport of mitochondria by either KBD or R₂KR₃ is highly coordinated, because their kinetic effects are accompanied always by changes in mitochondrial motile events of either transport polarity. These studies uncover novel roles in Ran GTPase-independent subdomains of RBD₂ and RBD₃, and KBD of RanBP2, that confer antagonizing and multi-modal mechanisms of kinesin-1 activation and regulation of mitochondrial motility. These findings open new venues towards the pharmacological harnessing of cooperative and competitive mechanisms regulating kinesins, RanBP2 or mitochondrial motility in disparate human disorders.

The ubiquitin-proteasome system in retinal health and disease

The ubiquitin-proteasome system (UPS) is the main intracellular pathway for modulated protein turnover, playing an important role in the maintenance of cellular homeostasis. It also exerts a protein quality control through degradation of oxidized, mutant, denatured, or misfolded proteins and is involved in many biological processes where protein level regulation is necessary. This system allows the cell to modulate its protein expression pattern in response to changing physiological conditions and provides a critical protective role in health and disease. Impairments of UPS function in the central nervous system (CNS) underlie an increasing number of genetic and idiopathic diseases, many of which affect the retina. Current knowledge on the UPS composition and function in this tissue, however, is scarce and dispersed. This review focuses on UPS elements reported in the retina, including ubiquitinating and deubiquitinating enzymes (DUBs), and alternative proteasome assemblies. Known and inferred roles of protein ubiquitination, and of the related, SUMO conjugation (SUMOylation) process, in normal retinal development and adult homeostasis are addressed, including modulation of the visual cycle and response to retinal stress and injury. Additionally, the relationship between UPS dysfunction and human neurodegenerative disorders affecting the retina, including Alzheimer's, Parkinson's, and Huntington's diseases, are dealt with, together with numerous instances of retina-specific illnesses with UPS involvement, such as retinitis pigmentosa, macular degenerations, glaucoma, diabetic retinopathy (DR), and aging-related impairments. This information, though still basic and limited, constitutes a suitable framework to be expanded in incoming years and should prove orientative toward future therapy design targeting sight-affecting diseases with a UPS underlying basis.

Dynamics and diverse functions of nuclear pore complex proteins

Nuclear pore complexes (NPCs) are best known for their central role in controlling the molecular trafficking between the cytoplasm and the nucleus. NPCs are assembled from about 30 different proteins and a growing body of evidence suggests that these nucleoporins are not only acting in the context of NPCs, but also in the nucleoplasm and cytoplasm. In this context it is well accepted that a set of nucleoporins are important regulators of a variety of mitotic processes, including kinetochore assembly, spindle checkpoint control and cytokinesis, whereas others associate with chromatin and administer gene expression. However, the functional importance of nucleoporins go far beyond these roles and this review will provide an overview of the latest insights into the versatility of metazoan nucleoporins with an emphasis on their roles in cell migration, cellular signaling and tissue-specific activities.

From Drosophila to humans: reflections on the roles of the prolyl isomerases and chaperones, cyclophilins, in cell function and disease

Despite remarkable advances in human genetics and other genetic model systems, the fruit fly, Drosophila melanogaster, remains a powerful experimental tool to probe with ease the inner workings of a myriad of biological and pathological processes, even when evolutionary forces impart apparent divergences to some of such processes. The understanding of such evolutionary differences provides mechanistic insights into genotype-phenotype correlations underpinning biological processes across metazoans. The pioneering work developed by the William Pak laboratory for the past four decades, and the work of others, epitomize the notion of how the Drosophila system breaks new fertile ground or complements research fields of high scientific and medical relevance. Among the three major genetic complementation groups produced by the Pak's laboratory and impairing distinct facets of photoreceptor neuronal function, the nina group (ninaA, …., ninaJ) selectively affects the biogenesis of G protein-coupled receptors (GPCRs), mediating the photoconversion and transduction of light stimuli. Among the nina genes identified, ninaA arguably assumes heightened significance for several reasons. First, it presents unique physiological selectivity toward the biogenesis of a subset of GPCRs, a standalone biological manifestation yet to be discerned for most mammalian homologues of NinaA. Second, NinaA belongs to a family of proteins, immunophilins, which are the primary targets for immunosuppressive drugs at the therapeutic forefront of a multitude of medical conditions. Third, NinaA closest homologue, cyclophilin B (CyPB/PPIB), is an immunophilin whose loss-of-function was found recently to cause osteogenesis imperfecta in the human. This report highlights advances made by studies on some members of immunophilins, the cyclophilins. Finally, it reexamines critically data and dogmas derived from past and recent genetic, structural, biological, and pathological studies on NinaA and few other cyclophilins that support some of such paradigms to be less than definite and advance our understanding of the roles of cyclophilins in cell function, disease, and therapeutic interventions.

HIV-1 capsid-cyclophilin interactions determine nuclear import pathway, integration targeting and replication efficiency

Lentiviruses such as HIV-1 traverse nuclear pore complexes (NPC) and infect terminally differentiated non-dividing cells, but how they do this is unclear. The cytoplasmic NPC protein Nup358/RanBP2 was identified as an HIV-1 co-factor in previous studies. Here we report that HIV-1 capsid (CA) binds directly to the cyclophilin domain of Nup358/RanBP2. Fusion of the Nup358/RanBP2 cyclophilin (Cyp) domain to the tripartite motif of TRIM5 created a novel inhibitor of HIV-1 replication, consistent with an interaction in vivo. In contrast to CypA binding to HIV-1 CA, Nup358 binding is insensitive to inhibition with cyclosporine, allowing contributions from CypA and Nup358 to be distinguished. Inhibition of CypA reduced dependence on Nup358 and the nuclear basket protein Nup153, suggesting that CypA regulates the choice of the nuclear import machinery that is engaged by the virus. HIV-1 cyclophilin-binding mutants CA G89V and P90A favored integration in genomic regions with a higher density of transcription units and associated features than wild type virus. Integration preference of wild type virus in the presence of cyclosporine was similarly altered to regions of higher transcription density. In contrast, HIV-1 CA alterations in another patch on the capsid surface that render the virus less sensitive to Nup358 or TRN-SR2 depletion (CA N74D, N57A) resulted in integration in genomic regions sparse in transcription units. Both groups of CA mutants are impaired in replication in HeLa cells and human monocyte derived macrophages. Our findings link HIV-1 engagement of cyclophilins with both integration targeting and replication efficiency and provide insight into the conservation of viral cyclophilin recruitment.

The nucleoporin RanBP2 tethers the cAMP effector Epac1 and inhibits its catalytic activity

Direct interaction between the catalytic domain of Epac1 and the nuclear pore component RanBP2 blocks Epac1 catalytic activity and downstream cAMP signaling.

Cyclic adenosine monophosphate (cAMP) is a second messenger that relays a wide range of hormone responses. In this paper, we demonstrate that the nuclear pore component RanBP2 acts as a negative regulator of cAMP signaling through Epac1, a cAMP-regulated guanine nucleotide exchange factor for Rap. We show that Epac1 directly interacts with the zinc fingers (ZNFs) of RanBP2, tethering Epac1 to the nuclear pore complex (NPC). RanBP2 inhibits the catalytic activity of Epac1 in vitro by binding to its catalytic CDC25 homology domain. Accordingly, cellular depletion of RanBP2 releases Epac1 from the NPC and enhances cAMP-induced Rap activation and cell adhesion. Epac1 also is released upon phosphorylation of the ZNFs of RanBP2, demonstrating that the interaction can be regulated by posttranslational modification. These results reveal a novel mechanism of Epac1 regulation and elucidate an unexpected link between the NPC and cAMP signaling.

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.

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.

Biology and biophysics of the nuclear pore complex and its components

Nucleocytoplasmic exchange of proteins and ribonucleoprotein particles occurs via nuclear pore complexes (NPCs) that reside in the double membrane of the nuclear envelope (NE). Significant progress has been made during the past few years in obtaining better structural resolution of the three-dimensional architecture of NPC with the help of cryo-electron tomography and atomic structures of domains from nuclear pore proteins (nucleoporins). Biophysical and imaging approaches have helped elucidate how nucleoporins act as a selective barrier in nucleocytoplasmic transport. Nucleoporins act not only in trafficking of macromolecules but also in proper microtubule attachment to kinetochores, in the regulation of gene expression and signaling events associated with, for example, innate and adaptive immunity, development and neurodegenerative disorders. Recent research has also been focused on the dynamic processes of NPC assembly and disassembly that occur with each cell cycle. Here we review emerging results aimed at understanding the molecular arrangement of the NPC and how it is achieved, defining the roles of individual nucleoporins both at the NPC and at other sites within the cell, and finally deciphering how the NPC serves as both a barrier and a conduit of active transport.

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.

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.

Heterologous expression of limulus rhodopsin

Invertebrates such as Drosophila or Limulus assemble their visual pigment into the specialized rhabdomeric membranes of photoreceptors where phototransduction occurs. We have investigated the biosynthesis of rhodopsin from the Limulus lateral eye with three cell culture expression systems: mammalian COS1 cells, insect Sf9 cells, and amphibian Xenopus oocytes. We extracted and affinity-purified epitope-tagged Limulus rhodopsin expressed from a cDNA or cRNA from these systems. We found that all three culture systems could efficiently synthesize the opsin polypeptide in quantities comparable with that found for bovine opsin. However, none of the systems expressed a protein that stably bound 11-cis-retinal. The protein expressed in COS1 and Sf9 cells appeared to be misfolded, improperly localized, and proteolytically degraded. Similarly, Xenopus oocytes injected with Limulus opsin cRNA did not evoke light-sensitive currents after incubation with 11-cis-retinal. However, injecting Xenopus oocytes with mRNA from Limulus lateral eyes yielded light-dependent conductance changes after incubation with 11-cis-retinal. Also, expressing Limulus opsin cDNA in the R1-R6 photoreceptors of transgenic Drosophila yielded a visual pigment that bound retinal, had normal spectral properties, and coupled to the endogenous phototransduction cascade. These results indicate that Limulus opsin may require one or more photoreceptor-specific proteins for correct folding and/or chromophore binding. This may be a general property of invertebrate opsins and may underlie some of the functional differences between invertebrate and vertebrate visual pigments.

Trafficking prerogatives of olfactory receptors

Olfactory receptors lead lives of exclusivity and privilege, the monarchs of fiefdoms organized solely to carry out their instructions. Each olfactory sensory neuron expresses one allele of one of approximately 1000 olfactory receptor genes. It is thought that olfactory receptor diversity is critical for the ability of animals to detect many thousands of odorants, but supporting functional evidence has been difficult to obtain because olfactory receptors expressed in heterologous cells are typically retained in the endoplasmic reticulum. The membrane trafficking entitlements enjoyed by olfactory receptors appear to be available only in mature olfactory sensory neurons. Evidence is accumulating that cell-type-specific accessory proteins regulate first the exit of olfactory receptors from the endoplasmic reticulum, and then the trafficking of olfactory receptors from post-Golgi compartments to the plasma membrane of the olfactory cilia where the receptors gain access to odorants. Critical olfactory receptor accessory proteins are known only in the nematode Caenorhabditis elegans, where the absence of a novel protein called ODR-4 or a clathrin adaptor, UNC-101, interferes with proper trafficking. Similar functional specificity also occurs in a parallel chemosensory system, the mammalian vomeronasal organ. Trafficking of the V2R type of vomeronasal receptors is mediated by a vomeronasal-specific family of major histocompatibility complex proteins. Removal of olfactory receptors from the plasma membrane may be regulated in a more conventional fashion because odor stimulation has been linked to receptor phosphorylation, to the function of G-protein coupled receptor kinase 3, and to an increase in vesicles retrieved from the plasma membrane.

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.

Unfolding retinal dystrophies: a role for molecular chaperones?

Inherited retinal dystrophy is a major cause of blindness worldwide. Recent molecular studies have suggested that protein folding and molecular chaperones might play a major role in the pathogenesis of these degenerations. Incorrect protein folding could be a common consequence of causative mutations in retinal degeneration disease genes, particularly mutations in the visual pigment rhodopsin. Furthermore, several retinal degeneration disease genes have recently been identified as putative facilitators of correct protein folding, molecular chaperones, on the basis of sequence homology. We also consider whether manipulation of chaperone levels or chaperone function might offer potential novel therapies for retinal degeneration.

Olfactory receptor trafficking involves conserved regulatory steps

Olfactory receptors are difficult to functionally express in heterologous cells. They are typically retained in the endoplasmic reticulum of cells commonly used for functional expression studies and are only released to the plasma membrane in mature cells of the olfactory receptor neuron lineage. A recently developed olfactory cell line, odora, traffics olfactory receptors to the plasma membrane when differentiated. We found that undifferentiated odora cells do not traffic olfactory receptors to their surface, even though they release the receptors to the Golgi apparatus and endosomes. This behavior differs from other cell lines tested thus far. Differentiated odora cells also properly traffic vomeronasal receptors of the VN1 type, which lack sequence similarity to olfactory receptors. ODR-4, a protein that is necessary for plasma membrane trafficking of a chemosensory receptor in nematodes, facilitates trafficking of rat olfactory receptor U131 in odora and Chinese hamster ovary cells. Olfactory receptor trafficking from the endoplasmic reticulum to the plasma membrane involves at least two steps whose regulation depends on the maturation state of cells in the olfactory receptor neuron lineage. These results also indicate that some components of the regulatory mechanism are conserved.

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.

Large-scale production and purification of functional recombinant bovine rhodopsin with the use of the baculovirus expression system

Here we describe a generic procedure for the expression and purification of milligram quantities of functional recombinant eukaryotic integral membrane proteins, exemplified by hexahistidine-tagged bovine rhodopsin. These quantities were obtained with the recombinant baculovirus/Sf9 insect cell-based expression system in large-scale bioreactor cultures with the use of a serum-free and protein-free growth medium. After optimization procedures, expression levels up to 4 mg/l were established. The recombinant rhodopsin could be purified with high overall yield by using immobilized-metal-affinity chromatography on Ni(2+)-agarose. After reconstitution into a native lipid environment, the purified protein was functionally indistinguishable from native rhodopsin with regard to the following parameters: spectral absorbance band, structural changes after photoactivation, and G-protein activation. The procedures developed can be adapted to other membrane proteins. The ability to produce and purify tens of milligrams of functional recombinant eukaryotic membrane protein meets the ever-increasing demand of material necessary to perform detailed biochemical and structural biophysical studies that are essential in unravelling their working mechanism at a molecular level.

Truncation releases olfactory receptors from the endoplasmic reticulum of heterologous cells

Olfactory receptors are difficult to express functionally in heterologous cells. We found that olfactory receptors traffic poorly to the plasma membrane even in cells with neuronal phenotypes, including cell lines derived from the olfactory epithelium. Other than mature olfactory receptor neurons, few cells appear able to traffic olfactory receptors to the plasma membrane. In human embryonic kidney 293 cells and Xenopus fibroblasts, olfactory receptor immunoreactivity overlapped with a marker for the endoplasmic reticulum (ER) but not with markers for the Golgi apparatus or endosomes. Except for the ER, olfactory receptors were therefore absent from organelles normally involved in the plasma membrane trafficking of receptors. Olfactory receptors truncated prior to transmembrane domain VI were expressed in the plasma membrane, however. Co-expression of the missing C-terminal fragment with these truncated receptors prevented their expression in the plasma membrane. Intramolecular interactions between N- and C-terminal domains joined by the third cytoplasmic loop appear to be responsible for retention of olfactory receptors in the ER of heterologous cells. Our results are consistent with misfolding of the receptors but could also be explained by altered trafficking of the receptors.

Native Escherichia coli and murine dihydrofolate reductases contain late-folding non-native structures

We have examined the equilibrium and kinetic folding properties of two structurally homologous dihydrofolate reductases, Escherichia coli DHFR (EcDHFR) and murine DHFR (MuDHFR), as a function of temperature and ligand concentration. Conformational heterogeneity in native DHFR is well documented, and the results demonstrate that the non-native form(s) represents late intermediate(s) in the folding process. We have measured the concentrations of native and non-native forms and the rate constants for their interconversion over a temperature range of 3 degreesC to 49 degreesC, allowing characterization of the thermodynamic as well as the kinetic properties of the final folding step(s) relative to the overall folding reaction. Differences in ligand binding suggest that the intermediate structures for these two proteins may be different during refolding.

The cyclophilin-like domain mediates the association of Ran-binding protein 2 with subunits of the 19 S regulatory complex of the proteasome

The combination of the Ran-binding domain 4 and cyclophilin domains of Ran-binding protein 2 selectively associate with a subset of G protein-coupled receptors, red/green opsins, upon cis-trans prolyl isomerase-dependent and direct modification of opsin followed by association of the modified opsin isoform to Ran-binding domain 4. This effect enhances in vivo the production of functional receptor and generates an opsin isoform with no propensity to self-aggregate in vitro. We now show that another domain of Ran-binding protein 2, cyclophilin-like domain, specifically associates with the 112-kDa subunit, P112, and other subunits of the 19 S regulatory complex of the 26 S proteasome in the neuroretina. This association possibly mediates Ran-binding protein 2 limited proteolysis into a smaller and stable isoform. Also, the interaction of Ran-binding protein 2 with P112 regulatory subunit of the 26 S proteasome involves still another protein, a putative kinesin-like protein. Our results indicate that Ran-binding protein 2 is a key component of a macro-assembly complex selectively linking protein biogenesis with the proteasome pathway and, thus, with potential implications for the presentation of misfolded and ubiquitin-like modified proteins to this proteolytic machinery.

Detailed characterization of a cyclophilin from the human malaria parasite Plasmodium falciparum

Cyclosporin (Cs) A has pronounced antimalarial activity in vitro and in vivo. In other organisms, the drug is thought to exert its effects either by inhibiting the peptidylprolyl cis/trans isomerase activity of cyclophilin (CyP) or by forming a CyP-CsA complex that inhibits the phosphatase activity of calcineurin. We have cloned and overexpressed in Escherichia coli a gene encoding a CyP from Plasmodium falciparum (PfCyP19) that is located on chromosome 3. The sequence of PfCyP19 shows remarkable sequence identity with human CyPA and, unlike the two previously identified CyPs from P. falciparum, PfCyP19 has no signal peptide or N-terminal sequence extension, suggesting a cytosolic localization. All the residues implicated in the recognition of the synthetic substrate N-succinyl-Ala-Ala-Pro-Phe-p-nitroanilide are conserved, resulting in characteristically high Michaelis-Menten and specificity constants (Km>>120 microM, kcat/Km=1.2x10(7) M-1.s-1 respectively). As the first line in the functional characterization of this enzyme, we have assessed its binding affinity for CsA. In accordance with its tryptophan-containing CsA-binding domain, PfCyP19 binds CsA with high affinity (Kd=13 nM, Ki=6.9 nM). Twelve CsA analogues were also found to possess Ki values similar to CsA, with the notable exceptions of Val2-Cs (Ki=218 nM) and Thr2-Cs (Ki=690 nM). The immunosuppressants rapamycin and FK506 were inactive as inhibitors, consistent with other members of the CyP family of rotamases. The CsA analogues were also assessed as inhibitors of P. falciparum growth in vitro. Although their antimalarial activity did not correlate with inhibition of enzyme activity, residues 3 and 4 of CsA appeared to be important for inhibition of parasite growth and residues 1 and 2 for PfCyP19 inhibition. We propose that a malarial CyP-CsA complex presents residues 3 and 4 as part of an 'effector surface' for recognition by a downstream target, similar to the proposed mechanism for T-cell immunosuppression.

Large-scale production and purification of the human green cone pigment: characterization of late photo-intermediates

We present the first characterization of the late photo-intermediates (Meta I, Meta II and Meta III) of a vertebrate cone pigment in a lipid environment. Marked differences from the same pathway in the rod pigment were observed. The histidine-tagged human green cone pigment was functionally expressed in large-scale suspension cultures in Sf9 insect cells using recombinant baculovirus. The recombinant pigment was extensively purified in a single step by immobilized metal affinity chromatography and displays the expected spectral characteristics. The purified pigment was able to activate the rod G-protein transducin at about half the rate of the rod pigment. Following reconstitution into bovine retina lipid proteoliposomes, identification and analysis of the photo-intermediates Meta I, Meta II and Meta III was accomplished. Similar to the rod pigment, our results indicate the existence of a Meta I-Meta II equilibrium, but we find no evidence for pH dependence. Replacement of native Cl- by NO3- in the anion-binding site of the cone pigment affected the spectral position of the pigment itself and of the Meta I intermediate, but not that of Meta II and Meta III. The decay rate of the 'active' intermediate Meta II did not differ for the Cl- and NO3- state. However, in qualitative agreement with results reported before for chicken cone pigments, the rate of Meta II decay was significantly higher in the human cone pigment than in the rod pigment.

Identification of a novel Ran binding protein 2 related gene (RANBP2L1) and detection of a gene cluster on human chromosome 2q11-q12

The giant 358-kDa protein Ran binding protein 2 (RanBP2/Nup358) is localized at the cytoplasmic side of the nuclear pore complex and likely constitutes the Ran-GTP binding site at the cytoplasmic face of the complex. RanBP2/Nup358 furthermore acts as a chaperone for red/green opsin molecules. Here, we report on the physical mapping of human RanBP2 between markers D2S340 and D2S1893. A duplication of the 5'-end sequence of RanBP2 occurs within 3 Mb distal to RanBP2. Detailed sequence analysis resulted in primers specific for this distal duplication. Polymerase chain reaction-based screening of cDNA libraries indicates that this transcript, called RanBP2alpha (HGMW-approved symbol RANBP2L1), is expressed in several tissues. Screening of a fetal brain cDNA library yielded a 4057-bp partial cDNA clone for RanBP2alpha. Its 5'-end is almost identical to RanBP2, whereas its 3'-part is distinct from RanBP2. Northern blot analysis using a probe of the 3'-untranslated sequence of RanBP2alpha detected in several tissues an 8-kb transcript representing the full length of the transcript. In pancreas and placenta, an additional transcript of 14 kb was detected. PAC clones containing the bona fide RanBP2 sequences were localized to 2q11-q12 by FISH analysis, and a region of high similarity was detected on 2p11-p12. In summary, we have identified a RanBP2 gene cluster on 2q11-q12 together with a novel gene termed RanBP2alpha, with high sequence similarity to RanBP2.