Rab proteins

A review on evolution, structural characteristics, interactions, and regulation of the membrane transport protein: The family of Rab proteins

Rab proteins are a key family of small GTPases that play crucial roles in vesicular trafficking, membrane dynamics, and maintaining cellular homeostasis. Studying this family of proteins is interesting as having many structural isoforms with variable evolutionary trends and wide distribution in cells. The proteins are renowned for their unique structural characteristics, which support their functional adaptability and specificity. Based on these features these proteins show different regulatory pathways and show involvement in dynamic protein-protein interactions, which is essential for intracellular signaling processes and in maintaining cellular functionality and balance. Notably, it is the first review to compile such extensive information about Rabs. Such information related to these proteins explores the molecular mechanisms in medicine based on their phylogenetic development, structural conformation changes, interaction networks, distribution, and regulation-dysregulations discussed in this review. Moreover, this review offers a consolidated resource for researchers and clinicians to understand the Rabs in different magnitudes.

Pleiotropic effects on Sarcopenia subphenotypes point to potential molecular markers for the disease

Sarcopenia is a progressive age-related muscle disease characterized by low muscle strength, quantity and quality, and low physical performance. The clinical overlap between these subphenotypes (reduction in muscle strength, quantity and quality, and physical performance) was evidenced, but the genetic overlap is still poorly investigated. Herein, we investigated whether there is a genetic overlap amongst sarcopenia subphenotypes in the search for more effective molecular markers for this disease. For that, a Bioinformatics approach was used to identify and characterize pleiotropic effects at the genome, loci and gene levels using Genome-wide association study results. As a result, a high genetic correlation was identified between gait speed and muscle strength (rG=0.5358, p=3.39 × 10-8). Using a Pleiotropy-informed conditional and conjunctional false discovery rate method we identified two pleiotropic loci for muscle strength and gait speed, one of them was nearby the gene PHACTR1. Moreover, 11 pleiotropic loci and 25 genes were identified for muscle mass and muscle strength. Lastly, using a gene-based GWAS approach three candidate genes were identified in the overlap of the three Sarcopenia subphenotypes: FTO, RPS10 and CALCR. The current study provides evidence of genetic overlap and pleiotropy among sarcopenia subphenotypes and highlights novel candidate genes and molecular markers associated with the risk of sarcopenia.

Citrulline facilitates the glycolysis, proliferation, and metastasis of lung cancer cells by regulating RAB3C

Lung cancer (LC) is one of the major malignant diseases threatening human health. The study aimed to identify the effect of citrulline on the malignant phenotype of LC cells and to further disclose the potential molecular mechanism of citrulline in regulating the development of LC, providing a novel molecular biological basis for the clinical treatment of LC. The effects of citrulline on the viability, proliferation, migration, and invasion of LC cells (A549, H1299) were validated by CCK-8, colony formation, EdU, and transwell assays. The cell glycolysis was assessed via determining the glucose uptake, lactate production, ATP levels, extracellular acidification rate (ECAR), and oxygen consumption rate (OCR). RNA-seq and molecular docking were performed to screen for citrulline-binding target proteins. Western blotting experiments were conducted to examine the expression of related signaling pathway molecules. In addition, the impacts of citrulline on LC growth in vivo were investigated by constructing mouse models. Citrulline augmented the viability of LC cells in a concentration and time-dependent manner. The proliferation, migration, invasion, glycolysis, and EMT processes of LC cells were substantially enhanced after citrulline treatment. Bioinformatics analysis indicated that citrulline could bind to RAB3C protein. Western blotting results indicated that citrulline activated the IL-6/STAT3 pathway by binding to RAB3C. In addition, animal experiments disclosed that citrulline promoted tumor growth in mice. Citrulline accelerated the glycolysis and activated the IL6/STAT3 pathway through the RAB3C protein, consequently facilitating the development of LC.

RAB11A and RAB11B control mitotic spindle function in intestinal epithelial progenitor cells

RAB11 small GTPases and associated recycling endosome have been localized to mitotic spindles and implicated in regulating mitosis. However, the physiological significance of such regulation has not been observed in mammalian tissues. We have used newly engineered mouse models to investigate intestinal epithelial renewal in the absence of single or double isoforms of RAB11 family members: Rab11a and Rab11b. Comparing with single knockouts, mice with compound ablation demonstrate a defective cell cycle entry and robust mitotic arrest followed by apoptosis, leading to a total penetrance of lethality within 3 days of gene ablation. Upon Rab11 deletion ex vivo, enteroids show abnormal mitotic spindle formation and cell death. Untargeted proteomic profiling of Rab11a and Rab11b immunoprecipitates has uncovered a shared interactome containing mitotic spindle microtubule regulators. Disrupting Rab11 alters kinesin motor KIF11 function and impairs bipolar spindle formation and cell division. These data demonstrate that RAB11A and RAB11B redundantly control mitotic spindle function and intestinal progenitor cell division, a mechanism that may be utilized to govern the homeostasis and renewal of other mammalian tissues.

Zika Virus Strains and Dengue Virus Induce Distinct Proteomic Changes in Neural Stem Cells and Neurospheres

Brain abnormalities and congenital malformations have been linked to the circulating strain of Zika virus (ZIKV) in Brazil since 2016 during the microcephaly outbreak; however, the molecular mechanisms behind several of these alterations and differential viral molecular targets have not been fully elucidated. Here we explore the proteomic alterations induced by ZIKV by comparing the Brazilian (Br ZIKV) and the African (MR766) viral strains, in addition to comparing them to the molecular responses to the Dengue virus type 2 (DENV). Neural stem cells (NSCs) derived from induced pluripotent stem (iPSCs) were cultured both as monolayers and in suspension (resulting in neurospheres), which were then infected with ZIKV (Br ZIKV or ZIKV MR766) or DENV to assess alterations within neural cells. Large-scale proteomic analyses allowed the comparison not only between viral strains but also regarding the two- and three-dimensional cellular models of neural cells derived from iPSCs, and the effects on their interaction. Altered pathways and biological processes were observed related to cell death, cell cycle dysregulation, and neurogenesis. These results reinforce already published data and provide further information regarding the biological alterations induced by ZIKV and DENV in neural cells.

Role of the Mosaic Cisternal Maturation Machinery in Glycan Synthesis and Oncogenesis

Tumorigenesis is associated with the deregulation of multiple processes, among which the glycosylation of lipids and proteins is one of the most extensively affected. However, in most cases, it remains unclear whether aberrant glycosylation is a cause, a link in the pathogenetic chain, or a mere consequence of tumorigenesis. In other cases, instead, studies have shown that aberrant glycans can promote oncogenesis. To comprehend how aberrant glycans are generated it is necessary to clarify the underlying mechanisms of glycan synthesis at the Golgi apparatus, which are still poorly understood. Important factors that determine the glycosylation potential of the Golgi apparatus are the levels and intra-Golgi localization of the glycosylation enzymes. These factors are regulated by the process of cisternal maturation which transports the cargoes through the Golgi apparatus while retaining the glycosylation enzymes in the organelle. This mechanism has till now been considered a single, house-keeping and constitutive function. Instead, we here propose that it is a mosaic of pathways, each controlling specific set of functionally related glycosylation enzymes. This changes the conception of cisternal maturation from a constitutive to a highly regulated function. In this new light, we discuss potential new groups oncogenes among the cisternal maturation machinery that can contribute to aberrant glycosylation observed in cancer cells. Further, we also discuss the prospects of novel anticancer treatments targeting the intra-Golgi trafficking process, particularly the cisternal maturation mechanism, to control/inhibit the production of pro-tumorigenic glycans.

Identification of Rab family genes and functional analyses of LmRab5 and LmRab11A in the development and RNA interference of Locusta migratoria

Rab proteins constitute the largest family of small GTPases, which play pivotal roles in intracellular membrane trafficking in all eukaryotes. A number of Rab genes have been identified in eukaryotes; however, very little information about these genes has been reported in insects. In the current study, for the first time we identified and characterized 27 Rab family genes from Locusta migratoria. Phylogenetic analysis and comparison of domain architecture indicated that Rab family genes are highly conserved among insect species. Tissue-dependent expression profiles indicated that expression of Rab genes was highest in the ovary, except for LmRab3, which was most highly expressed in hemolymph. The biological function of each Rab gene was investigated using RNA interference (RNAi). Double-stranded RNA targeting each Rab gene was injected into the hemocoel of nymphs and revealed that suppression of two Rab genes (LmRab5 and LmRab11A) caused 100% mortality. In addition, nymphs injected with dsLmRab5 exhibited severe phenotypic defects in the gastric caeca and midgut, while dsLmRab11A arrested the molting process. We then applied the RNAi of RNAi technique to test if silencing either of these two genes would affect the suppression of the lethal giant larvae (LmLgl) reporter gene and found that suppression of LmRab5 diminished the RNAi efficiency of LmLgl, whereas suppression of LmRab11A enhanced RNAi efficiency of LmLgl. These results indicate that Rab genes contribute differently to RNAi efficiency in different tissues. Our study provides a foundation for further functional investigations of Rab genes and their contributions to RNAi efficiency in L. migratoria.

Establishment and analysis of conditional Rab1- and Rab5-knockout cells using the auxin-inducible degron system

Two small GTPases, Rab1 and Rab5, are key membrane trafficking regulators that are conserved in all eukaryotes. They have recently been found to be essential for cell survival and/or growth in cultured mammalian cells, thereby precluding the establishment of Rab1-knockout (KO) and Rab5-KO cells, making it extremely difficult to assess the impact of complete Rab1 or Rab5 protein depletion on cellular functions. Here, we generated and analyzed cell lines with conditional KO (CKO) of either Rab1 (Rab1A and Rab1B) or Rab5 (Rab5A, Rab5B and Rab5C) by using the auxin-inducible protein degradation system. Rab1 CKO and Rab5 CKO led to eventual cell death from 18 h and 48 h, respectively, after auxin exposure. After acute Rab1 protein depletion, the Golgi stack and ribbon structures were completely disrupted, and endoplasmic reticulum (ER)-to-Golgi trafficking was severely inhibited. Moreover, we discovered a novel Rab1-depletion phenotype: perinuclear clustering of early endosomes and delayed transferrin recycling. In contrast, acute Rab5 protein depletion resulted in loss of early endosomes and late endosomes, but lysosomes appeared to be normal. We also observed a dramatic reduction in the intracellular signals of endocytic cargos via receptor-mediated or fluid-phase endocytosis in Rab5-depleted cells.

A modified density gradient proteomic-based method to analyze endolysosomal proteins in cardiac tissue

The importance of lysosomes in cardiac physiology and pathology is well established, and evidence for roles in calcium signaling is emerging. We describe a label-free proteomics method suitable for small cardiac tissue biopsies based on density-separated fractionation, which allows study of endolysosomal (EL) proteins. Density gradient fractions corresponding to tissue lysate; sarcoplasmic reticulum (SR), mitochondria (Mito) (1.3 g/mL); and EL with negligible contamination from SR or Mito (1.04 g/mL) were analyzed using Western blot, enzyme activity assay, and liquid chromatography with tandem mass spectrometry (LC-MS/MS) analysis (adapted discontinuous Percoll and sucrose differential density gradient). Kyoto Encyclopedia of Genes and Genomes, Reactome, Panther, and Gene Ontology pathway analysis showed good coverage of RAB proteins and lysosomal cathepsins (including cardiac-specific cathepsin D) in the purified EL fraction. Significant EL proteins recovered included catalytic activity proteins. We thus present a comprehensive protocol and data set of guinea pig atrial EL organelle proteomics using techniques also applicable for non-cardiac tissue.

Knockin' on Egg's Door: Maternal Control of Egg Activation That Influences Cortical Granule Exocytosis in Animal Species

Fertilization by multiple sperm leads to lethal chromosomal number abnormalities, failed embryo development, and miscarriage. In some vertebrate and invertebrate eggs, the so-called cortical reaction contributes to their activation and prevents polyspermy during fertilization. This process involves biogenesis, redistribution, and subsequent accumulation of cortical granules (CGs) at the female gamete cortex during oogenesis. CGs are oocyte- and egg-specific secretory vesicles whose content is discharged during fertilization to block polyspermy. Here, we summarize the molecular mechanisms controlling critical aspects of CG biology prior to and after the gametes interaction. This allows to block polyspermy and provide protection to the developing embryo. We also examine how CGs form and are spatially redistributed during oogenesis. During egg activation, CG exocytosis (CGE) and content release are triggered by increases in intracellular calcium and relies on the function of maternally-loaded proteins. We also discuss how mutations in these factors impact CG dynamics, providing unprecedented models to investigate the genetic program executing fertilization. We further explore the phylogenetic distribution of maternal proteins and signaling pathways contributing to CGE and egg activation. We conclude that many important biological questions and genotype–phenotype relationships during fertilization remain unresolved, and therefore, novel molecular players of CG biology need to be discovered. Future functional and image-based studies are expected to elucidate the identity of genetic candidates and components of the molecular machinery involved in the egg activation. This, will open new therapeutic avenues for treating infertility in humans.

Isolation and Proteomics of the Insulin Secretory Granule

Insulin, a vital hormone for glucose homeostasis is produced by pancreatic beta-cells and when secreted, stimulates the uptake and storage of glucose from the blood. In the pancreas, insulin is stored in vesicles termed insulin secretory granules (ISGs). In Type 2 diabetes (T2D), defects in insulin action results in peripheral insulin resistance and beta-cell compensation, ultimately leading to dysfunctional ISG production and secretion. ISGs are functionally dynamic and many proteins present either on the membrane or in the lumen of the ISG may modulate and affect different stages of ISG trafficking and secretion. Previously, studies have identified few ISG proteins and more recently, proteomics analyses of purified ISGs have uncovered potential novel ISG proteins. This review summarizes the proteins identified in the current ISG proteomes from rat insulinoma INS-1 and INS-1E cell lines. Here, we also discuss techniques of ISG isolation and purification, its challenges and potential future directions.

Thiazole-Based Thiosemicarbazones: Synthesis, Cytotoxicity Evaluation and Molecular Docking Study

INTRODUCTION

Hybrid drug design has developed as a prime method for the development of novel anticancer therapies that can theoretically solve much of the pharmacokinetic disadvantages of traditional anticancer drugs. Thus a number of studies have indicated that thiazole-thiophene hybrids and their bis derivatives have important anticancer activity. Mammalian Rab7b protein is a member of the Rab GTPase protein family that controls the trafficking from endosomes to the TGN. Alteration in the Rab7b expression is implicated in differentiation of malignant cells, causing cancer.

METHODS

1-(4-Methyl-2-(2-(1-(thiophen-2-yl) ethylidene) hydrazinyl) thiazol-5-yl) ethanone was used as building block for synthesis of novel series of 5-(1-(2-(thiazol-2-yl) hydrazono) ethyl) thiazole derivatives. The bioactivities of the synthesized compounds were evaluated with respect to their antitumor activities against MCF-7 tumor cells using MTT assay. Computer-aided docking protocol was performed to study the possible molecular interactions between the newly synthetic thiazole compounds and the active binding site of the target protein Rab7b. Moreover, the in silico prediction of adsorption, distribution, metabolism, excretion (ADME) and toxicity (T) properties of synthesized compounds were carried out using admetSAR tool.

RESULTS

The results obtained showed that derivatives 9 and 11b have promising activity (IC50 = 14.6 ± 0.8 and 28.3 ± 1.5 µM, respectively) compared to Cisplatin (IC50 = 13.6 ± 0.9 µM). The molecular docking analysis reveals that the synthesized compounds are predicted to be fit into the binding site of the target Rab7b. In summary, the synthetic thiazole compounds 1-17 could be used as potent inhibitors as anticancer drugs.

CONCLUSION

Promising anticancer activity of compounds 9 and 11 compared with cisplatin reference drug suggests that these ligands may contribute as lead compounds in search of new anticancer agents to combat chemo-resistance.

Discovery of a Role for Rab3b in Habituation and Cocaine Induced Locomotor Activation in Mice Using Heterogeneous Functional Genomic Analysis

Substance use disorders are prevalent and present a tremendous societal cost but the mechanisms underlying addiction behavior are poorly understood and few biological treatments exist. One strategy to identify novel molecular mechanisms of addiction is through functional genomic experimentation. However, results from individual experiments are often noisy. To address this problem, the convergent analysis of multiple genomic experiments can discern signal from these studies. In the present study, we examine genetic loci that modulate the locomotor response to cocaine identified in the recombinant inbred (BXD RI) genetic reference population. We then applied the GeneWeaver software system for heterogeneous functional genomic analysis to integrate and aggregate multiple studies of addiction genomics, resulting in the identification of Rab3b as a functional correlate of the locomotor response to cocaine in rodents. This gene encodes a member of the RAB family of Ras-like GTPases known to be involved in trafficking of secretory and endocytic vesicles in eukaryotic cells. The convergent evidence for a role of Rab3b includes co-occurrence in previously published genetic mapping studies of cocaine related behaviors; methamphetamine response and cocaine- and amphetamine-regulated transcript prepropeptide (Cartpt) transcript abundance; evidence related to other addictive substances; density of polymorphisms; and its expression pattern in reward pathways. To evaluate this finding, we examined the effect of RAB3 complex perturbation in cocaine response. B6;129-Rab3btm1Sud Rab3ctm1sud Rab3dtm1sud triple null mice (Rab3bcd -/-) exhibited significant deficits in habituation, and increased acute and repeated cocaine responses. This previously unidentified mechanism of the behavioral predisposition and response to cocaine is an example of many that can be identified and validated using aggregate genomic studies.

Stimulatory effect on the transport mediated by organic anion transporting polypeptide 2B1

Drug-drug interaction (DDI) is one of causes of adverse drug events and can result in life-threatening consequences. Organic anion-transporting polypeptide (OATP) 2B1 is a major uptake transporter in the intestine and contributes to transport various clinically used therapeutic agents. The intestine has a high risk of DDI, because it has a special propensity to be exposed to a high concentration of drugs. Thus, understanding drug interaction mediated by OATP2B1 in the absorption process is important for the prevention of adverse drug events, including decrease in the therapeutic effect of co-administered drugs. Acute drug interaction occurs through the direct inhibitory effect on transporters, including OATP2B1. Moreover, some compounds such as clinically used drugs and food components have an acute stimulatory effect on transport of co-administered drugs by OATP2B1. This review summarizes the acute stimulatory effect on the transport mediated by OATP2B1 and discusses the mechanisms of the acute stimulatory effects of compounds. There are two types of acute stimulatory effects, substrate-independent and -dependent interactions on OATP2B1 function. The facilitating translocation of OATP2B1 to the plasma membrane is one of causes for the substrate-independent acute stimulatory effect. On the contrary, the substrate-dependent effect is based on the direct binding to the substrate-binding site or allosteric progesterone-binding site of OATP2B1.

Rab24 functions in meiotic apparatus assembly and maturational progression in mouse oocyte

Rab GTPases have multiple regulatory functions in intracellular vesicle transport. In recent years, there has been an increasing interest in the roles of Rab proteins in mammalian oocytes. In this paper, we show the specific distribution pattern of Rab24 during mouse oocyte meiosis. Furthermore, we find that Rab24 depletion results in the failure of maturational progression in mouse oocytes. Notably, the frequency of meiotic apparatus abnormality is significantly increased in Rab24-depleted oocytes relative to controls. In addition, lagging chromosomes are readily observed in anaphase/telophase oocytes with Rab24 knockdown. In support of this, the depletion of Rab24 disturbs the kinetochore-microtubule attachments in oocytes, and contributes to the production of aneuploid eggs. Taken together, the results of this study identify Rab24 as a novel factor in the modulation of meiotic apparatus assembly and meiotic progression during mouse oocyte maturation.

Regulation of RabGAPs involved in insulin action

Rab (Ras-related proteins in brain) GTPases are key proteins responsible for a multiplicity of cellular trafficking processes. Belonging to the family of monomeric GTPases, they are regulated by cycling between their active GTP-bound and inactive GDP-bound conformations. Despite possessing a slow intrinsic GTP hydrolysis activity, Rab proteins rely on RabGAPs (Rab GTPase-activating proteins) that catalyze GTP hydrolysis and consequently inactivate the respective Rab GTPases. Two related RabGAPs, TBC1D1 and TBC1D4 (=AS160) have been described to be associated with obesity-related traits and type 2 diabetes in both mice and humans. Inactivating mutations of TBC1D1 and TBC1D4 lead to substantial changes in trafficking and subcellular distribution of the insulin-responsive glucose transporter GLUT4, and to subsequent alterations in energy substrate metabolism. The activity of the RabGAPs is controlled through complex phosphorylation events mediated by protein kinases including AKT and AMPK, and by putative regulatory interaction partners. However, the dynamics and downstream events following phosphorylation are not well understood. This review focuses on the specific role and regulation of TBC1D1 and TBC1D4 in insulin action.

Proteomic Analysis of Cerebral Cortex Extracts from Sus scrofa with Induced Hemorrhagic Stroke

Cerebrovascular diseases, including stroke and micro stroke, are the main causes of death in contemporary society. Hemorrhagic stroke is the fast emerging defficiency in the brain function resulting from disturbance of blood supply to the brain caused by the rapture of blood vessels (Lopez et al. in Proteomics Clin Appl 6:190-200, 2012). The influence of a model hemorrhagic stroke on white pigs with the change in the protein profile of their cortical samples 24 h and 2 months after the stroke was examined using mass-spectrometric analysis. Different proteins (n = 30) were identified, and their content was elevated. These proteins are involved in the mechanisms of neuroprotection, including compensation of oxidative stress (TXN, SNCA, PRDX6, ENO1), prevention of unwanted protein aggregation and apoptosis (PTMA, SNCA, SNCB), release of neurotransmitters (GAPDH, PEBP1) and assembly of the cytoskeleton (ACTA2, PTMA, TUBA4A, TUBA1D), etc. Also, a group of seven Ras family proteins involved in the regulation of cell proliferation and differentiation was found in the samples taken 24 h following the stroke. The relative concentrations of most of the proteins in the samples taken 2 months after the stroke demonstrate intermediate values between the control sample and the sample taken in 24 h, indicating the extinction of change in the protein profile with time. During the first 24 h after the stroke, there is an increase in protein fractions participating in exocytosis, synaptic plasticity/signaling, and support of neurotransmitter transport. Such shift in the weight of protein functional clusters can be attributed to activation of compensatory mechanisms in the body focused on neuroprotection.

Role of Rab GTPases in the export trafficking of G protein-coupled receptors

G protein-coupled receptors (GPCRs) constitute a superfamily of cell surface receptors that regulate a variety of cell functions. As the cell surface is the functional destination for most GPCRs, the cell surface targeting process represents a crucial checkpoint in controlling the functionality of the receptors. However, the molecular mechanisms underlying the cell surface delivery of newly synthesized GPCRs remain poorly understood. In this review, we will highlight the role of Rab GTPases in GPCR cell surface transport, particularly post-Golgi traffic, and discuss the underlying molecular mechanisms.

Yersinia pestis Targets the Host Endosome Recycling Pathway during the Biogenesis of the Yersinia-Containing Vacuole To Avoid Killing by Macrophages

Yersinia pestis has evolved many strategies to evade the innate immune system. One of these strategies is the ability to survive within macrophages. Upon phagocytosis, Y. pestis prevents phagolysosome maturation and establishes a modified compartment termed the Yersinia-containing vacuole (YCV). Y. pestis actively inhibits the acidification of this compartment, and eventually, the YCV transitions from a tight-fitting vacuole into a spacious replicative vacuole. The mechanisms to generate the YCV have not been defined. However, we hypothesized that YCV biogenesis requires Y. pestis interactions with specific host factors to subvert normal vesicular trafficking. In order to identify these factors, we performed a genome-wide RNA interference (RNAi) screen to identify host factors required for Y. pestis survival in macrophages. This screen revealed that 71 host proteins are required for intracellular survival of Y. pestis. Of particular interest was the enrichment for genes involved in endosome recycling. Moreover, we demonstrated that Y. pestis actively recruits Rab4a and Rab11b to the YCV in a type three secretion system-independent manner, indicating remodeling of the YCV by Y. pestis to resemble a recycling endosome. While recruitment of Rab4a was necessary to inhibit YCV acidification and lysosomal fusion early during infection, Rab11b appeared to contribute to later stages of YCV biogenesis. We also discovered that Y. pestis disrupts global host endocytic recycling in macrophages, possibly through sequestration of Rab11b, and this process is required for bacterial replication. These data provide the first evidence that Y. pestis targets the host endocytic recycling pathway to avoid phagolysosomal maturation and generate the YCV.

Yersinia pestis can infect and survive within macrophages. However, the mechanisms that the bacterium use to subvert killing by these phagocytes have not been defined. To provide a better understanding of these mechanisms, we used an RNAi approach to identify host factors required for intracellular Y. pestis survival. This approach revealed that the host endocytic recycling pathway is essential for Y. pestis to avoid clearance by the macrophage. We further demonstrate that Y. pestis remodels the phagosome to resemble a recycling endosome, allowing the bacterium to avoid the normal phagolysosomal maturation pathway. Moreover, we show that infection with Y. pestis disrupts normal recycling in the macrophage and that disruption is required for bacterial replication. These findings provide the first evidence that Y. pestis targets the host endocytic recycling pathway in order to evade killing by macrophages.

Molecular Insights into the Roles of Rab Proteins in Intracellular Dynamics and Neurodegenerative Diseases

In eukaryotes, the cellular functions are segregated to membrane-bound organelles. This inherently requires sorting of metabolites to membrane-limited locations. Sorting the metabolites from ribosomes to various organelles along the intracellular trafficking pathways involves several integral cellular processes, including an energy-dependent step, in which the sorting of metabolites between organelles is catalyzed by membrane-anchoring protein Rab-GTPases (Rab). They contribute to relaying the switching of the secretory proteins between hydrophobic and hydrophilic environments. The intracellular trafficking routes include exocytic and endocytic pathways. In these pathways, numerous Rab-GTPases are participating in discrete shuttling of cargoes. Long-distance trafficking of cargoes is essential for neuronal functions, and Rabs are critical for these functions, including the transport of membranes and essential proteins for the development of axons and neurites. Rabs are also the key players in exocytosis of neurotransmitters and recycling of neurotransmitter receptors. Thus, Rabs are critical for maintaining neuronal communication, as well as for normal cellular physiology. Therefore, cellular defects of Rab components involved in neural functions, which severely affect normal brain functions, can produce neurological complications, including several neurodegenerative diseases. In this review, we provide a comprehensive overview of the current understanding of the molecular signaling pathways of Rab proteins and the impact of their defects on different neurodegenerative diseases. The insights gathered into the dynamics of Rabs that are described in this review provide new avenues for developing effective treatments for neurodegenerative diseases-associated with Rab defects.

Genome-wide characterization of the Rab gene family in Gossypium by comparative analysis

BACKGROUND

Rab protein family is the largest subfamily of small G protein family. As one of the most important families in plant, Rab family plays an important role in the process of plant growth and development. So far, the identification of 57 members of the Rab family in Arabidopsis has been completed. In cotton, the relevant family has not been reported.

RESULTS

Here, we identified 87, 169, 136, 80 Rabs in the four sequenced cotton species, G. raimondii (D₅), G. hirsutum acc. TM-1 (AD₁), G. barbadense acc. 3-79 (AD₂) and G. arboreum (A₂), respectively. Biological information analysis showed that the number of amino acid is 200-300 aa among Rab family members in G. raimondii and the protein molecular weight is between 20 and 30 kDa, which is consistent with the characterization of the Rab protein itself. 87 GrRabs in G. raimondii are divided into eight groups. In each group, intron numbers and subcellular localization of Rab protein are basically the same. We mapped the distribution of GrRab genes on 13 chromosomes of G. raimondii except two genes. Among the 87 GrRabs in G. raimondii, we identified 60 pairs of GrRabs formed in whole genome duplication. Among all the gene pairs, the Ka/Ks values were less than 1. This indicates that it is the results of the purification selection and will help maintain the conservation of gene in structure and function. Further, 4 of the 87 GrRabs showed tandem duplication. They were GrRabA2a vs GrRabD1a and GrRabA2h vs GrRabD1b respectively. Expression patterns analysis of 169 GhRabs in G. hirsutum acc. TM-1 indicates that most Rab family members play a certain role in different tissues/organs and different growth stages of cotton, implying their potential function in the polar growth of pollen tube, root hair and fiber cell, as well as improving stress and disease tolerance.

CONCLUSION

The systematic investigation of Rab genes in cotton will lay a foundation for understanding the functional roles of different Rab members in the polar growth and stress tolerance.

Timely regulated sorting from early to late endosomes is required to maintain cerebellar long-term depression

An important feature of long-term synaptic plasticity is the prolonged maintenance of plastic changes in synaptic transmission. The trafficking of AMPA-type glutamate receptors (AMPARs) is involved in the expression of many forms of synaptic plasticity, yet the subsequent events accomplishing the maintenance of plastic changes in synaptic AMPAR numbers are not fully understood. Here, we find that maintenance of cerebellar long-term depression results from a reduction in the number of AMPARs residing within endocytic recycling pathways. We then develop a genetically encoded, photosensitive inhibitor of late endosome sorting and use this to discover that initial maintenance of long-term depression relies on timely regulated late endosome sorting, which exhibits a threshold as well as switch-like behavior. Thus, our results indicate that recycling AMPAR numbers are reduced by a switching machinery of transient late endosome sorting, and that this process enables the transition from basal synaptic transmission to long-term depression maintenance.

Long term depression (LTD) of the cerebellum is known to be mediated by postsynaptic trafficking of glutamate receptor AMPAR. Here, Kim and colleagues show that early- to late-endosomal sorting of AMPAR represents the switch from expression to maintenance phase of cerebellar LTD.

Modulation of Retrograde Trafficking of KCa3.1 in a Polarized Epithelium

In epithelia, the intermediate conductance, Ca²⁺-activated K⁺ channel (KCa3.1) is targeted to the basolateral membrane (BLM) where this channel plays numerous roles in absorption and secretion. A growing body of research suggests that the membrane resident population of KCa3.1 may be critical in clinical manifestation of diseases. In this study, we investigated the key molecular components that regulate the degradation of KCa3.1 using a Fisher rat thyroid cell line stably expressing KCa3.1. Using immunoblot, Ussing chamber, and pharmacological approaches, we demonstrated that KCa3.1 is targeted exclusively to the BLM, provided a complete time course of degradation of KCa3.1 and degradation time courses of the channel in the presence of pharmacological inhibitors of ubiquitylation and deubiquitylation to advance our understanding of the retrograde trafficking of KCa3.1. We provide a complete degradation profile of KCa3.1 and that the degradation is via an ubiquitin-dependent pathway. Inhibition of E1 ubiquitin activating enzyme by UBEI-41 crippled the ability of the cells to internalize the channel, shown by the increased BLM surface expression resulting in an increased function of the channel as measured by a DCEBIO sensitive K⁺ current. Additionally, the involvement of deubiquitylases and degradation by the lysosome were also confirmed by treating the cells with PR-619 or leupeptin/pepstatin, respectively; which significantly decreased the degradation rate of membrane KCa3.1. Additionally, we provided the first evidence that KCa3.1 channels were not deubiquitylated at the BLM. These data further define the retrograde trafficking of KCa3.1, and may provide an avenue for therapeutic approach for treatment of disease.

Getting membrane proteins on and off the shuttle bus between the endoplasmic reticulum and the Golgi complex

Secretory proteins exit the endoplasmic reticulum (ER) in coat protein complex II (COPII)-coated vesicles and then progress through the Golgi complex before delivery to their final destination. Soluble cargo can be recruited to ER exit sites by signal-mediated processes (cargo capture) or by bulk flow. For membrane proteins, a third mechanism, based on the interaction of their transmembrane domain (TMD) with lipid microdomains, must also be considered. In this Commentary, I review evidence in favor of the idea that partitioning of TMDs into bilayer domains that are endowed with distinct physico-chemical properties plays a pivotal role in the transport of membrane proteins within the early secretory pathway. The combination of such self-organizational phenomena with canonical intermolecular interactions is most likely to control the release of membrane proteins from the ER into the secretory pathway.

Early events in hepatitis B virus infection: From the cell surface to the nucleus

While most adults are able to clear acute hepatitis B virus (HBV) infection, chronic HBV infection is recalcitrant to current therapy because of the persistence of covalently closed circular DNA in the nucleus. Complete clearance of the virus in these patients is rare, and long-term therapy with interferon and/or nucleoside analogues may be required in an attempt to suppress viral replication and prevent progressive liver damage. The difficulty of establishing HBV infection in cell culture and experimental organisms has hindered efforts to elucidate details of the HBV life cycle, but it has also revealed the importance of the cellular microenvironment required for HBV binding and entry. Recent studies have demonstrated an essential role of sodium-taurocholate cotransporting polypeptide as a functional receptor in HBV infection, which has facilitated the development of novel infection systems and opened the way for more detailed understanding of the early steps of HBV infection as well as a potential new therapeutic target. However, many gaps remain in understanding of how HBV recognizes and attaches to hepatocytes prior to binding to sodium-taurocholate cotransporting polypeptide, as well as events that are triggered after binding, including entry into the cell, intracellular transport, and passage through the nuclear pore complex. This review summarizes current knowledge of the initial stages of HBV infection leading to the establishment of covalently closed circular DNA in the nucleus.

A complex of Rab13 with MICAL-L2 and α-actinin-4 is essential for insulin-dependent GLUT4 exocytosis

Rab13 is necessary for insulin-regulated GLUT4-vesicle exocytosis in muscle. Biochemical and imaging analyses provide evidence that activated Rab13 engages a scaffold protein MICAL-L2 to form a complex with Rab13 and α-actinin-4. Through GLUT4 interaction with α-actinin-4, GLUT4 vesicles are recruited to the muscle plasma membrane.

Insulin promotes glucose uptake into skeletal muscle through recruitment of glucose transporter 4 (GLUT4) to the plasma membrane. Rab GTPases are molecular switches mobilizing intracellular vesicles, and Rab13 is necessary for insulin-regulated GLUT4–vesicle exocytic translocation in muscle cells. We show that Rab13 engages the scaffold protein MICAL-L2 in this process. RNA interference–mediated knockdown of MICAL-L2 or truncated MICAL-L2 (MICAL-L2-CT) impaired insulin-stimulated GLUT4 translocation. Insulin increased Rab13 binding to MICAL-L2, assessed by pull down and colocalization under confocal fluorescence and structured illumination microscopies. Association was also visualized at the cell periphery using TIRF microscopy. Insulin further increased binding of MICAL-L2 to α-actinin-4 (ACTN4), a protein involved in GLUT4 translocation. Rab13, MICAL-L2, and ACTN4 formed an insulin-dependent complex assessed by pull down and confocal fluorescence imaging. Of note, GLUT4 associated with the complex in response to insulin, requiring the ACTN4-binding domain in MICAL-L2. This was demonstrated by pull down with distinct fragments of MICAL-L2 and confocal and structured illumination microscopies. Finally, expression of MICAL-L2-CT abrogated the insulin-dependent colocalization of Rab13 with ACTN4 or Rab13 with GLUT4. Our findings suggest that MICAL-L2 is an effector of insulin-activated Rab13, which links to GLUT4 through ACTN4, localizing GLUT4 vesicles at the muscle cell periphery to enable their fusion with the membrane.

Yersinia pestis Requires Host Rab1b for Survival in Macrophages

Yersinia pestis is a facultative intracellular pathogen that causes the disease known as plague. During infection of macrophages Y. pestis actively evades the normal phagosomal maturation pathway to establish a replicative niche within the cell. However, the mechanisms used by Y. pestis to subvert killing by the macrophage are unknown. Host Rab GTPases are central mediators of vesicular trafficking and are commonly targeted by bacterial pathogens to alter phagosome maturation and killing by macrophages. Here we demonstrate for the first time that host Rab1b is required for Y. pestis to effectively evade killing by macrophages. We also show that Rab1b is specifically recruited to the Yersinia containing vacuole (YCV) and that Y. pestis is unable to subvert YCV acidification when Rab1b expression is knocked down in macrophages. Furthermore, Rab1b knockdown also altered the frequency of association between the YCV with the lysosomal marker Lamp1, suggesting that Rab1b recruitment to the YCV directly inhibits phagosome maturation. Finally, we show that Rab1b knockdown also impacts the pH of the Legionella pneumophila containing vacuole, another pathogen that recruits Rab1b to its vacuole. Together these data identify a novel role for Rab1b in the subversion of phagosome maturation by intracellular pathogens and suggest that recruitment of Rab1b to the pathogen containing vacuole may be a conserved mechanism to control vacuole pH.

Yersinia pestis is the bacterial agent that causes the human disease known as plague. While often considered a historic disease, Y. pestis is endemic in rodent populations on several continents and the World Health Organization considers plague to be a reemerging disease. Much of the success of this pathogen comes from its ability to evade clearance by the innate immune system of its host. One weapon in the Y. pestis arsenal is its ability to resist killing when engulfed by macrophages. Upon invasion of macrophages, Y. pestis actively manipulates the cell to generate a protective vacuolar compartment, called the Yersinia containing vacuole (YCV) that allows the bacterium to evade the normal pathogen killing mechanisms of the macrophage. Here we demonstrate that the host protein Rab1b is recruited to the YCV and is required for Y. pestis to inhibit both the acidification and normal maturation of the phagosome to establish a protective niche within the cell. Rab1b is the first protein, either from the host or Y. pestis, shown to contribute to the biogenesis of the YCV. Furthermore, our data suggest a previously unknown impact of Rab1b recruitment in the phagosome maturation pathway.

A single mutation in helix 8 enhances the angiotensin II type 1a receptor transport and signaling

The amphipathic helix 8 in the membrane-proximal C-terminus is a structurally conserved feature of class A seven transmembrane-spanning G protein-coupled receptors (GPCRs). Mutations of this helical motif often cause receptor misfolding, defective cell surface transport and dysfunction. Surprisingly, we demonstrated here that a single point mutation at Lys308 in helix 8 markedly enhanced the steady-state surface density of the angiotensin II type 1a receptor (AT1aR). Consistent with the enhanced cell surface expression, Lys308 mutation significantly augmented AT1aR-mediated mitogen-activated protein kinase ERK1/2 activation, inositol phosphate production, and vascular smooth muscle cell migration. This mutation also increased the overall expression of AT1aR without altering receptor degradation. More interestingly, Lys308 mutation abolished AT1aR interaction with β-COP, a component of COPI transport vesicles, and impaired AT1aR responsiveness to the inhibition of Rab6 GTPase involved in the Golgi-to-ER retrograde pathway. Furthermore, these functions of Lys308 were largely dependent on its positively charged property. These data reveal previously unappreciated functions of helix 8 and novel mechanisms governing the cell surface transport and function of AT1aR.

The trials and tubule-ations of Rab6 involvement in Golgi-to-ER retrograde transport

In the early secretory pathway, membrane flow in the anterograde direction from the endoplasmic reticulum (ER) to the Golgi complex needs to be tightly co-ordinated with retrograde flow to maintain the size, composition and functionality of these two organelles. At least two mechanisms of transport move material in the retrograde direction: one regulated by the cytoplasmic coatomer protein I complex (COPI), and a second COPI-independent pathway utilizing the small GTP-binding protein Rab6. Although the COPI-independent pathway was discovered 15 years ago, it remains relatively poorly characterized, with only a handful of machinery molecules associated with its operation. One feature that makes this pathway somewhat unusual, and potentially difficult to study, is that the transport carriers predominantly seem to be tubular rather than vesicular in nature. This suggests that the regulatory machinery is likely to be different from that associated with vesicular transport pathways controlled by conventional coat complexes. In the present mini-review, we have highlighted the key experiments that have characterized this transport pathway so far and also have discussed the challenges that lie ahead with respect to its further characterization.

HIV-1 endocytosis in astrocytes: a kiss of death or survival of the fittest?

The brain is a target of HIV-1 and serves as an important viral reservoir. Astrocytes, the most abundant glial cell in the human brain, are involved in brain plasticity and neuroprotection. Several studies have reported HIV-1 infection of astrocytes in cell cultures and infected brain tissues. The prevailing concept is that HIV-1 infection of astrocytes leads to latent infection. Here, we provide our perspective on endocytosis-mediated HIV-1 entry and its fate in astrocytes. Natural entry of HIV-1 into astrocytes occurs via endocytosis. However, endocytosis of HIV-1 in astrocytes is a natural death trap where the majority of virus particles are degraded in endosomes and a few which escape intact lead to successful infection. Thus, regardless of artificial fine-tuning (treatment with cytokines or proinflammatory products) done to astrocytes, HIV-1 does not infect them efficiently unless the viral entry route or the endosomal enzymatic machinery has been manipulated.

Small GTPase and regulation of inflammation response in atherogenesis

Small GTPases are key signal transducers from extracellular stimuli to the nucleus that regulate a variety of cellular responses, including changes in gene expression and cell adhesion and migration. Accumulating data have demonstrated that abnormal activation of these small GTPases plays a critical role in the atherosclerosis characterized by vascular abnormalities, especially endothelial dysfunction and inflammation. Here, we discuss the linkage between small GTPases, inflammation, and atherogenesis. First, small GTPases affect gene expression of inflammatory cytokines through proinflammatory signaling pathways, such as nuclear factor-κB, vascular cell adhesion molecule-1, intercellular adhesion molecule-1, interlukin-8, and monocyte chemoattractant protein-1. Then, these molecules regulate the vascular inflammation through cell adhesion and migration. In turn, small GTPases are also regulated by extracellular stimuli, such as L-selectin, thrombin, oxidized phospholipids, and interleukins. Thus, these inflammatory cytokines generate a vicious cycle for small GTPases and inflammatory responses in the atherogenesis.

Hsc70 contributes to cancer cell survival by preventing Rab1A degradation under stress conditions

Heat shock cognate protein 70 (Hsc70) acts as a molecular chaperone for the maintenance of intracellular proteins, which allows cancer cells to survive under proteotoxic stress. We attempted to use Hsc70 to identify key molecules in cancer cell survival. Here, we performed mass-spectrometry-based proteomics analysis utilizing affinity purification with anti-Hsc70 antibodies; as a result, 83 differentially expressed proteins were identified under stress conditions. This result implies that there was a change in the proteins with which Hsc70 interacted in response to stress. Among the proteins identified under both serum-depleted and 5-fluorouracil-treated conditions, Rab1A was identified as an essential molecule for cancer cell survival. Hsc70 interacted with Rab1A in a chaperone-dependent manner. In addition, Hsc70 knockdown decreased the level of Rab1A and increased the level of its ubiquitination under stress conditions, suggesting that Hsc70 prevented the degradation of Rab1A denatured by stress exposure. We also found that Rab1A knockdown induced cell death by inhibition of autophagosome formation. Rab1A may therefore contribute to overcoming proteotoxic insults, which allows cancer cells to survive under stress conditions. Analysis of Hsc70 interactors provided insight into changes of intracellular status. We expect further study of the Hsc70 interactome to provide a more comprehensive understanding of cancer cell physiology.

Endocytosis-mediated HIV-1 entry and its significance in the elusive behavior of the virus in astrocytes

Astrocytes protect neurons but also evoke a proinflammatory response to injury and viral infections including HIV. We investigated the mechanism of HIV-1 infection in primary astrocytes, which showed minimal but productive viral infection independent of CXCR4. As with ectopic-CD4-expressing astrocytes, lysosomotropic agents led to increased HIV-1 infection in wild-type but not Rabs 5, 7, and 11-ablated astrocytes. Instead, HIV-1 infection was decreased in Rab-depleted astrocytes, corroborating viral entry by endocytosis. HIV-1 produced persistent infection in astrocytes (160 days); no evidence of latent infection was seen. Notably, one caveat is that endosomal modifiers enhanced wild-type HIV-1 infection (M- and T-tropic) in astrocytes, suggesting endocytic entry of the virus. Impeding endocytosis by inhibition of Rab 5, 7 or 11 will inhibit HIV infection in astrocytes. Although the contribution of such low-level infection in astrocytes to neurological complications is unclear, it may serve as an elusive viral reservoir in the central nervous system.

Anterograde trafficking of KCa3.1 in polarized epithelia is Rab1- and Rab8-dependent and recycling endosome-independent

The intermediate conductance, Ca2+-activated K+ channel (KCa3.1) targets to the basolateral (BL) membrane in polarized epithelia where it plays a key role in transepithelial ion transport. However, there are no studies defining the anterograde and retrograde trafficking of KCa3.1 in polarized epithelia. Herein, we utilize Biotin Ligase Acceptor Peptide (BLAP)-tagged KCa3.1 to address these trafficking steps in polarized epithelia, using MDCK, Caco-2 and FRT cells. We demonstrate that KCa3.1 is exclusively targeted to the BL membrane in these cells when grown on filter supports. Following endocytosis, KCa3.1 degradation is prevented by inhibition of lysosomal/proteosomal pathways. Further, the ubiquitylation of KCa3.1 is increased following endocytosis from the BL membrane and PR-619, a deubiquitylase inhibitor, prevents degradation, indicating KCa3.1 is targeted for degradation by ubiquitylation. We demonstrate that KCa3.1 is targeted to the BL membrane in polarized LLC-PK1 cells which lack the μ1B subunit of the AP-1 complex, indicating BL targeting of KCa3.1 is independent of μ1B. As Rabs 1, 2, 6 and 8 play roles in ER/Golgi exit and trafficking of proteins to the BL membrane, we evaluated the role of these Rabs in the trafficking of KCa3.1. In the presence of dominant negative Rab1 or Rab8, KCa3.1 cell surface expression was significantly reduced, whereas Rabs 2 and 6 had no effect. We also co-immunoprecipitated KCa3.1 with both Rab1 and Rab8. These results suggest these Rabs are necessary for the anterograde trafficking of KCa3.1. Finally, we determined whether KCa3.1 traffics directly to the BL membrane or through recycling endosomes in MDCK cells. For these studies, we used either recycling endosome ablation or dominant negative RME-1 constructs and determined that KCa3.1 is trafficked directly to the BL membrane rather than via recycling endosomes. These results are the first to describe the anterograde and retrograde trafficking of KCa3.1 in polarized epithelia cells.

Molecular characterization and expression of Rab7 from Clonorchis sinensis and its potential role in autophagy

Accumulating evidences suggest that Rab7 GTPase is important for the normal progression of autophagy. However, the role of Rab7 GTPase in regulation of autophagy in Clonorchis sinensis is not known. In this study, a gene encoding Rab7 was isolated from C. sinensis adult cDNA. Recombinant CsRab7 was expressed and purified from Escherichia coli. CsRab7 transcripts were detected in the cDNA of adult worm, metacercaria, cercaria, and egg of C. sinensis, and were highly expressed in the metacercaria. Immunohistochemical localization results revealed that CsRab7 was specifically deposited on the vitellarium and eggs of adult worm. Furthermore, EGFP signal of CsRab7WT and the active mutant CsRab7Q67L were associated with autophagic vesicles in transiently transfected 293T cells. It is concluded from the present study that CsRab7 GTPase possibly contributes to the development of C. sinensis and that the autophagy pathway could be an important site of action with respect to the developmental role of CsRab7 in C. sinensis.

Regulation of hepatitis B virus infection by Rab5, Rab7, and the endolysosomal compartment

Despite important progress toward deciphering human hepatitis B virus (HBV) entry into host cells, many aspects of the early steps of the life cycle remained completely obscure. Following endocytosis, HBV must travel through the complex network of the endocytic pathway to reach the cell nucleus and initiate replication. In addition to guiding the viral particles to the replication site, the endosomal vesicles may play a crucial role in infection, providing the appropriate environment for virus uncoating and nucleocapsid release. In this work, we investigated the trafficking of HBV particles internalized in permissive cells. Expression of key Rab proteins, involved in specific pathways leading to different intracellular locations, was modulated in HepaRG cells, using a stable and inducible short hairpin RNA (shRNA) expression system. The trafficking properties of the newly developed cells were demonstrated by confocal microscopy and flow cytometry using specific markers. The results showed that HBV infection strongly depends on Rab5 and Rab7 expression, indicating that HBV transport from early to mature endosomes is required for a step in the viral life cycle. This may involve reduction of disulfide bond-linked envelope proteins, as alteration of the redox potential of the endocytic pathway resulted in inhibition of infection. Subcellular fractionation of HBV-infected cells showed that viral particles are further transported to lysosomes. Intriguingly, infection was not dependent on the lysosomal activity, suggesting that trafficking to this compartment is a "dead-end" route. Together, these data add to our understanding of the HBV-host cell interactions controlling the early stages of infection.

Rab24 is required for normal cell division

Rab24 is an atypical member of the Rab GTPase family whose distribution in interphase cells has been characterized; however, its function remains largely unknown. In this study, we have analyzed the distribution of Rab24 throughout cell division. We have observed that Rab24 was located at the mitotic spindle in metaphase, at the midbody during telophase and in the furrow during cytokinesis. We have also observed partial co-localization of Rab24 and tubulin and demonstrated its association to microtubules. Interestingly, more than 90% of transiently transfected HeLa cells with Rab24 presented abnormal nuclear connections (i.e., chromatin bridges). Furthermore, in CHO cells stably transfected with GFP-Rab24wt, we observed a large percentage of binucleated and multinucleated cells. In addition, these cells presented an extremely large size and multiple failures in mitosis, as aberrant spindle formation (metaphase), delayed chromosomes (telophase) and multiple cytokinesis. A marked increase in binucleated, multinucleated and multilobulated nucleus formation was observed in HeLa cells depleted of Rab24. We also present evidence that a fraction of Rab24 associates with microtubules. In addition, Rab24 knock down resulted in misalignment of chromosomes and abnormal spindle formation in metaphase leading to the appearance of delayed chromosomes during late telophase and failures in cytokinesis. Our findings suggest that an adequate level of Rab24 is necessary for normal cell division. In summary, Rab24 modulates several mitotic events, including chromosome segregation and cytokinesis, perhaps through the interaction with microtubules.

Human α-synuclein modulates vesicle trafficking through its interaction with prenylated Rab acceptor protein 1

α-Synuclein has been implicated in the pathogenesis of Parkinson's disease. Although it is highly conserved, its physiological function has not yet been elucidated in detail. In an effort to define the function of α-synuclein, interacting proteins were screened in phage display assays. Prenylated Rab acceptor protein 1 (PRA1) was identified as an interacting partner. A selective interaction between α-synuclein and PRA1 was confirmed by coimmunoprecipitation and GST pull-down assays. PRA1 and α-synuclein were colocalized in N2a neuronal cells. Cotransfection of α-synuclein and PRA1 caused vesicles to accumulate in the periphery of the cytosol in neuronal cells, suggesting that overexpression of α-synuclein hinders proper vesicle trafficking and recycling as a result of the interaction between α-synuclein and PRA1.

Structure of the Drosophila melanogaster Rab6 GTPase at 1.4 Å resolution

Rab6 is a small GTPase that belongs to the p21 Ras superfamily. It is involved in vesicle trafficking between the Golgi apparatus and endosomes/ER in eukaryotes. The GDP-bound inactive protein undergoes conformational changes when the nucleotide is exchanged to GTP, allowing Rab6 to interact with a variety of different effector proteins. To further understand how these changes affect downstream protein binding, the crystal structure of Rab6 from Drosophila melanogaster has been solved to 1.4 Å resolution, the highest resolution for a Rab6 structure to date. The crystals belonged to space group C2, with unit-cell parameters a=116.5, b=42.71, c=86.86 Å, α=90, β=133.12, γ=90°. The model was refined to an R factor of 14.5% and an Rfree of 17.3%.

Two sorting motifs, a ubiquitination motif and a tyrosine motif, are involved in HIV-1 and simian immunodeficiency virus Nef-mediated receptor endocytosis

HIV-1 and SIV Nef proteins downregulate cell surface CD4 and MHC class I (MHC-I) molecules of infected cells, which are necessary for efficient viral replication and pathogenicity. We previously reported that K144 in HIV-1 Nef is di-ubiquitinated, and K144R substitution impairs Nef-mediated CD4 downregulation. In this report, we extend the role of ubiquitination at this lysine residue from Nef-mediated CD4 downregulation to Nef-mediated MHC-I downregulation and from HIV Nef to SIV Nef. All HIV-1 Nef mutants that contain K144R substitution are inactive in MHC-I downregulation. Tested MHC-I alleles include HLA-ABC endogenously expressed and HLA-A2 exogenously expressed in Jurkat T cells. CD4 downregulation by SIV Nef involves K176 that aligns with K144 in HIV-1 Nef, as well as an N-terminal tyrosine motif Y28Y39 not present in HIV-1 Nef. Dual mutation at K176 and Y28Y39 completely impaired SIV Nef-mediated CD4 and MHC-I downregulation, whereas a single mutation at K176 or Y28Y39 did not. The involvement of tyrosine motif in SIV Nef-mediated CD4 and MHC-I downregulation prompted us to investigate a putative tyrosine motif (Y202Y/F203) in HIV-1 Nef that is conserved among HIV-1 species. Single mutation at the tyrosine motif Y202F203 in HIV-1 Nef (NA7) greatly impaired Nef-mediated CD4 downregulation, which is similar to what we observed previously with the single mutation at lysine K144. Thus, our study demonstrated that Nef-mediated receptor endocytosis involves the ubiquitination motif and tyrosine motif.

Hedgehog signaling update

In vertebrate hedgehog signaling, hedgehog ligands are processed to become bilipidated and then multimerize, which allows them to leave the signaling cell via Dispatched 1 and become transported via glypicans and megalin to the responding cells. Hedgehog then interacts with a complex of Patched 1 and Cdo/Boc, which activates endocytic Smoothened to the cilium. Patched 1 regulates the activity of Smoothened (1) via Vitamin D3, which inhibits Smoothened in the absence of hedgehog ligand or (2) via oxysterols, which activate Smoothened in the presence of hedgehog ligand. Hedgehog ligands also interact with Hip1, Patched 2, and Gas1, which regulate the range as well as the level of hedgehog signaling. In vertebrates, Smoothened is shortened at its C-terminal end and lacks most of the phosphorylation sites of importance in Drosophila. Cos2, also of importance in Drosophila, plays no role in mammalian transduction, nor do its homologs Kif7 and Kif27. The cilium may provide a function analogous to that of Cos2 by linking Smoothened to the modulation of Gli transcription factors. Disorders associated with the hedgehog signaling network follow, including nevoid basal cell carcinoma syndrome, holoprosencephaly, Smith-Lemli-Opitz syndrome, Greig cephalopolysyndactyly syndrome, Pallister-Hall syndrome, Carpenter syndrome, and Rubinstein-Taybi syndrome.

Rab1 small GTP-binding protein regulates cell surface trafficking of the human calcium-sensing receptor

The human calcium-sensing receptor (hCaR) is a family-3/C G-protein-coupled receptor that regulates Ca(2+) homeostasis by controlling parathyroid hormone secretion. Here we investigated the role of Rab1, a small GTP-binding protein that specifically regulates protein transport from the endoplasmic reticulum to the Golgi, in cell surface transport of the hCaR. Cell surface expression of hCaR transiently expressed in human embryonic kidney 293 cells was strongly augmented by coexpression of Rab1 and attenuated by disruption of endogenous Rab1 function by expression of the dominant-negative Rab1N124I mutant or depletion of Rab1 with small interfering RNA. Rab1N124I expression also partially attenuated cell surface expression and signaling response to gain-of-function mutants of hCaR with truncated carboxyl-terminal sequences at positions 895 and 903. These carboxyl-tail truncations are similar to a deletion between residues S895 and V1075 found in a patient family causing autosomal dominant hypocalcemia. In addition, coexpression with wild-type Rab1 increased cell surface expression of the loss-of-function missense mutation R185Q, located on the hCaR amino-terminal extracellular ligand-binding domain (ECD), which causes familial hypocalciuric hypercalcemia. Truncated hCaR variants containing either the ECD with the first transmembrane helix or only the ECD also display Rab1-dependent cell surface expression or secretion into the culture medium, respectively. These data reveal a role for Rab1 in hCaR trafficking from the endoplasmic reticulum to the Golgi that regulates receptor cell surface expression and thereby cell signaling responsiveness to extracellular calcium.

The role of G proteins in assembly and function of Kir3 inwardly rectifying potassium channels

Kir3 channels (also known as GIRK channels) are important regulators of electrical excitability in both cardiomyocytes and neurons. Much is known regarding the assembly and function of these channels and the roles that their interacting proteins play in controlling these events. Further, they are one of the best studied effectors of heterotrimeric G proteins in general and Gβγ subunits in particular. However, our understanding of the roles of multiple Gβγ binding sites on Kir3 channels is still rudimentary. We discuss potential roles for Gβγ in channel assembly and trafficking in addition to their known role in cellular signaling.

Multiple phosphatidylinositol 3-kinases regulate vaccinia virus morphogenesis

Poxvirus morphogenesis is a complex process that involves the successive wrapping of the virus in host cell membranes. We screened by plaque assay a focused library of kinase inhibitors for those that caused a reduction in viral growth and identified several compounds that selectively inhibit phosphatidylinositol 3-kinase (PI3K). Previous studies demonstrated that PI3Ks mediate poxviral entry. Using growth curves and electron microscopy in conjunction with inhibitors, we show that that PI3Ks additionally regulate morphogenesis at two distinct steps: immature to mature virion (IMV) transition, and IMV envelopment to form intracellular enveloped virions (IEV). Cells derived from animals lacking the p85 regulatory subunit of Type I PI3Ks (p85α^−/−β^−/−) presented phenotypes similar to those observed with PI3K inhibitors. In addition, VV appear to redundantly use PI3Ks, as PI3K inhibitors further reduce plaque size and number in p85α^−/−β^−/− cells. Together, these data provide evidence for a novel regulatory mechanism for virion morphogenesis involving phosphatidylinositol dynamics and may represent a new therapeutic target to contain poxviruses.

Rab8 interacts with distinct motifs in alpha2B- and beta2-adrenergic receptors and differentially modulates their transport

The molecular mechanism underlying the post-Golgi transport of G protein-coupled receptors (GPCRs) remains poorly understood. Here we determine the role of Rab8 GTPase, which modulates vesicular protein transport between the trans-Golgi network (TGN) and the plasma membrane, in the cell surface targeting of alpha(2B)- and beta(2)-adrenergic receptors (AR). Transient expression of GDP- and GTP-bound Rab8 mutants and short hairpin RNA-mediated knockdown of Rab8 more potently inhibited the cell surface expression of alpha(2B)-AR than beta(2)-AR. The GDP-bound Rab8(T22N) mutant attenuated ERK1/2 activation by alpha(2B)-AR, but not beta(2)-AR, and arrested alpha(2B)-AR in the TGN compartment. Co-immunoprecipitation revealed that both alpha(2B)-AR and beta(2)-AR physically interacted with Rab8 and glutathione S-transferase fusion protein pulldown assays demonstrated that Rab8 interacted with the C termini of both receptors. Interestingly, mutation of the highly conserved membrane-proximal C terminus dileucine motif selectively blocked beta(2)-AR interaction with Rab8, whereas mutation of residues Val(431)-Phe(432)-Asn(433)-Gln(434), Pro(447)-Trp(448), Gln(450)-Thr(451), and Trp(453) in the C terminus impaired alpha(2B)-AR interaction with Rab8. Furthermore, transport inhibition by Rab8(T22N) of a chimeric beta(2)-AR carrying the alpha(2B)-AR C terminus was similar to alpha(2B)-AR. These data provide strong evidence indicating that Rab8 GTPase interacts with distinct motifs in the C termini of alpha(2B)-AR and beta(2)-AR and differentially modulates their traffic from the TGN to the cell surface.