The leaden gene product is required with Rab27a to recruit myosin Va to melanosomes in melanocytes

A frameshift variant in the melanophilin gene is associated with loss of pigment from shed skin in ball pythons ( Python regius )

Melanophilin is a myosin adaptor required for transporting the pigment melanin within cells. Loss of melanophilin in fish, birds, and mammals causes pigmentation defects, but little is known about the role of melanophilin in non-avian reptiles. Here we show that a frameshift in the melanophilin gene in ball python ( P. regius ) is associated with loss of pigment from shed skin. This variant is predicted to remove the myosin-binding domain of melanophilin and thereby impair transport of melanin-containing organelles. Our study represents the first description of a melanophilin variant in a non-avian reptile and confirms the role of melanophilin across vertebrates.

High expression of Talin-1 is associated with tumor progression and recurrence in melanoma skin cancer patients

BACKGROUND

Talin-1 as a component of multi-protein adhesion complexes plays a role in tumor formation and migration in various malignancies. This study investigated Talin-1 in protein levels as a potential prognosis biomarker in skin tumors.

METHODS

Talin-1 was evaluated in 106 skin cancer (33 melanomas and 73 non-melanomas skin cancer (NMSC)) and 11 normal skin formalin-fixed paraffin-embedded (FFPE) tissue samples using immunohistochemical technique on tissue microarrays (TMAs). The association between the expression of Talin-1 and clinicopathological parameters, as well as survival outcomes, were assessed.

RESULTS

Our findings from data minings through bioinformatics tools indicated dysregulation of Talin-1 in mRNA levels for skin cancer samples. In addition, there was a statistically significant difference in Talin-1 expression in terms of intensity of staining, percentage of positive tumor cells, and H-score in melanoma tissues compared to NMSC (P = 0.001, P < 0.001, and P < 0.001, respectively). Moreover, high cytoplasmic expression of Talin-1 was found to be associated with significantly advanced stages (P = 0.024), lymphovascular invasion (P = 0.023), and recurrence (P = 0.006) in melanoma cancer tissues. Our results on NMSC showed a statistically significant association between high intensity of staining and the poor differentiation (P = 0.044). No significant associations were observed between Talin-1 expression levels and survival outcomes of melanoma and NMSC patients.

CONCLUSION

Our observations showed that higher expression of Talin1 in protein level may be significantly associated with more aggressive tumor behavior and advanced disease in patients with skin cancer. However, further studies are required to find the mechanism of action of Talin-1 in skin cancers.

Overlapping Machinery in Lysosome-Related Organelle Trafficking: A Lesson from Rare Multisystem Disorders

Lysosome-related organelles (LROs) are a group of functionally diverse, cell type-specific compartments. LROs include melanosomes, alpha and dense granules, lytic granules, lamellar bodies and other compartments with distinct morphologies and functions allowing specialised and unique functions of their host cells. The formation, maturation and secretion of specific LROs are compromised in a number of hereditary rare multisystem disorders, including Hermansky-Pudlak syndromes, Griscelli syndrome and the Arthrogryposis, Renal dysfunction and Cholestasis syndrome. Each of these disorders impacts the function of several LROs, resulting in a variety of clinical features affecting systems such as immunity, neurophysiology and pigmentation. This has demonstrated the close relationship between LROs and led to the identification of conserved components required for LRO biogenesis and function. Here, we discuss aspects of this conserved machinery among LROs in relation to the heritable multisystem disorders they associate with, and present our current understanding of how dysfunctions in the proteins affected in the disease impact the formation, motility and ultimate secretion of LROs. Moreover, we have analysed the expression of the members of the CHEVI complex affected in Arthrogryposis, Renal dysfunction and Cholestasis syndrome, in different cell types, by collecting single cell RNA expression data from the human protein atlas. We propose a hypothesis describing how transcriptional regulation could constitute a mechanism that regulates the pleiotropic functions of proteins and their interacting partners in different LROs.

Novel RAB27A Variant Associated with Late-Onset Hemophagocytic Lymphohistiocytosis Alters Effector Protein Binding

Autosomal recessive mutations in RAB27A are associated with Griscelli syndrome type 2 (GS2), characterized by hypopigmentation and development of early-onset, potentially fatal hemophagocytic lymphohistiocytosis (HLH). We describe a 35-year old male who presented with recurrent fever, was diagnosed with Epstein-Barr virus-driven chronic lymphoproliferation, fulfilled clinical HLH criteria, and who carried a novel homozygous RAB27A c.551G > A p.(R184Q) variant. We aimed to evaluate the contribution of the identified RAB27A variant in regard to the clinical phenotype as well as cellular and biochemical function. The patient displayed normal pigmentation as well as RAB27A expression in blood-derived cells. However, patient NK and CD8+ T cell exocytosis was low. Ectopic expression of the RAB27A p.R184Q variant rescued melanosome distribution in mouse Rab27a-deficient melanocytes, but failed to increase exocytosis upon reconstitution of human RAB27A-deficient CD8+ T cells. Mechanistically, the RAB27A p.R184Q variant displayed reduced binding to SLP2A but augmented binding to MUNC13-4, two key effector proteins in immune cells. MUNC13-4 binding was particularly strong to an inactive RAB27A p.T23N/p.R184Q double mutant. RAB27A p.R184Q was expressed and could facilitate melanosome trafficking, but did not support lymphocyte exocytosis. The HLH-associated RAB27A variant increased Munc13-4 binding, potentially representing a novel mode of impairing RAB27A function selectively in hematopoietic cells.

Identification of a Novel MLPH Missense Mutation in a Chinese Griscelli Syndrome 3 Patient

Melanophilin (MLPH) functions as a linker between RAB27A and myosin Va (MYO5A) in regulating skin pigmentation during the melanosome transport process. The MYO5A-MLPH-RAB27A ternary protein complex is required for anchoring mature melanosomes in the peripheral actin filaments of melanocytes for subsequent transfer to adjacent keratinocytes. Griscelli syndrome type 3 (GS3) is caused by mutations in the MLPH gene. So far, only five variants of MLPH associated with GS3 have been reported. Here, we reported the first patient with GS3 in a Chinese population. The proband carried a novel homozygous missense mutation (c.73G&gt;C; p.D25H), residing in the conserved Slp homology domain of MLPH, and presented with hypopigmentation of the hair, eyebrows, and eyelashes. Light microscopy revealed the presence of abnormal pigment clumping in his hair shaft. In silico tools predicted this MLPH variant to be likely pathogenic. Using immunoblotting and immunofluorescence analysis, we demonstrated that the MLPH (D25H) variant had an inhibitory effect on melanosome transport by exhibiting perinuclear melanosome aggregation in melanocytes, and greatly reduced its binding to RAB27A, although the protein level of MLPH in the patient was not changed. Our findings suggest that MLPH (D25H) is a pathogenic variant that expands the genetic spectrum of the MLPH gene.

Current methods to analyze lysosome morphology, positioning, motility and function

Since the discovery of lysosomes more than 70 years ago, much has been learned about the functions of these organelles. Lysosomes were regarded as exclusively degradative organelles, but more recent research has shown that they play essential roles in several other cellular functions, such as nutrient sensing, intracellular signalling and metabolism. Methodological advances played a key part in generating our current knowledge about the biology of this multifaceted organelle. In this review, we cover current methods used to analyze lysosome morphology, positioning, motility and function. We highlight the principles behind these methods, the methodological strategies and their advantages and limitations. To extract accurate information and avoid misinterpretations, we discuss the best strategies to identify lysosomes and assess their characteristics and functions. With this review, we aim to stimulate an increase in the quantity and quality of research on lysosomes and further ground-breaking discoveries on an organelle that continues to surprise and excite cell biologists.

Diagnostic and therapeutic caveats in Griscelli syndrome

Griscelli syndrome (GS) is a rare autosomal recessive disease with characteristic pigment distribution, and there are currently 3 types according to the underlying genetic defect and clinical features. We present the case of a girl born from consanguineous parents who presented with predominant neurologic symptoms, silvery hair and granulomatous skin lesions. Cerebral magnetic resonance revealed diffuse white matter lesions, and central nervous system (CNS) lymphocytic infiltration was suspected. The patient underwent haematopoietic stem cell transplantation with graft failure and autologous reconstitution. She developed elevated liver enzyme with a cholestatic pattern. Multiple liver biopsies revealed centrilobular cholestasis and unspecific portal inflammation that improved with immunomodulatory treatment. She was revealed to have an impaired cytotoxicity in NK cells and a decreased expression of RAB27A. However, no variants were found in the gene. All types of GS present with pigment dilution and irregular pigment clumps that can be seen through light microscopy in hair and skin biopsy. Dermic granulomas and immunodeficiency with infectious and HLH predisposition have been described in GS type 2 (GS2). Neurologic alterations might be seen in GS type 1 (GS1) and GS type 2 (GS2), due to different mechanisms. GS1 presents with neurologic impairment secondary to myosin Va role in neuronal development and synapsis. Meanwhile, GS2 can present with neurologic impairment secondary to SNC HLH. Clinical features and cytotoxicity might aid in differentiating GS1 and GS2, especially since treatment differs.

Melanin Distribution in Human Skin: Influence of Cytoskeletal, Polarity, and Centrosome-Related Machinery of Stratum basale Keratinocytes

Melanin granules cluster within supra-nuclear caps in basal keratinocytes (KCs) of the human epidermis, where they protect KC genomic DNA against ultraviolet radiation (UVR) damage. While much is known about melanogenesis in melanocytes (MCs) and a moderate amount about melanin transfer from MC to KC, we know little about the fate of melanin once inside KCs. We recently reported that melanin fate in progenitor KCs is regulated by rare asymmetric organelle movement during mitosis. Here, we explore the role of actin, microtubules, and centrosome-associated machinery in distributing melanin within KCs. Short-term cultures of human skin explants were treated with cytochalasin-B and nocodazole to target actin filaments and microtubules, respectively. Treatment effects on melanin distribution were assessed by the Warthin-Starry stain, on centrosome-associated proteins by immunofluorescence microscopy, and on co-localisation with melanin granules by brightfield microscopy. Cytochalasin-B treatment disassembled supra-nuclear melanin caps, while nocodazole treatment moved melanin from the apical to basal KC domain. Centrosome and centriolar satellite-associated proteins showed a high degree of co-localisation with melanin. Thus, once melanin granules are transferred to KCs, their preferred apical distribution appears to be facilitated by coordinated movement of centrosomes and centriolar satellites. This mechanism may control melanin's strategic position within UVR-exposed KCs.

LRRK2-phosphorylated Rab10 sequesters Myosin Va with RILPL2 during ciliogenesis blockade

Pathogenic LRRK2 phosphorylation of Rab10 GTPase dramatically redistributes Myosin Va and RILPL2 proteins to the mother centriole and sequesters Myosin Va at that location in a manner that likely interferes with its role in ciliogenesis.

Activating mutations in LRRK2 kinase causes Parkinson’s disease. Pathogenic LRRK2 phosphorylates a subset of Rab GTPases and blocks ciliogenesis. Thus, defining novel phospho-Rab interacting partners is critical to our understanding of the molecular basis of LRRK2 pathogenesis. RILPL2 binds with strong preference to LRRK2-phosphorylated Rab8A and Rab10. RILPL2 is a binding partner of the motor protein and Rab effector, Myosin Va. We show here that the globular tail domain of Myosin Va also contains a high affinity binding site for LRRK2-phosphorylated Rab10. In the presence of pathogenic LRRK2, RILPL2 and MyoVa relocalize to the peri-centriolar region in a phosphoRab10-dependent manner. PhosphoRab10 retains Myosin Va over pericentriolar membranes as determined by fluorescence loss in photobleaching microscopy. Without pathogenic LRRK2, RILPL2 is not essential for ciliogenesis but RILPL2 over-expression blocks ciliogenesis in RPE cells independent of tau tubulin kinase recruitment to the mother centriole. These experiments show that LRRK2 generated-phosphoRab10 dramatically redistributes a significant fraction of Myosin Va and RILPL2 to the mother centriole in a manner that likely interferes with Myosin Va’s role in ciliogenesis.

Exophilin-5 regulates allergic airway inflammation by controlling IL-33-mediated Th2 responses

A common variant in the RAB27A gene in adults was recently found to be associated with the fractional exhaled nitric oxide level, a marker of eosinophilic airway inflammation. The small GTPase Rab27 is known to regulate intracellular vesicle traffic, although its role in allergic responses is unclear. We demonstrated that exophilin-5, a Rab27-binding protein, was predominantly expressed in both of the major IL-33 producers, lung epithelial cells, and the specialized IL-5 and IL-13 producers in the CD44hiCD62LloCXCR3lo pathogenic Th2 cell population in mice. Exophilin-5 deficiency increased stimulant-dependent damage and IL-33 secretion by lung epithelial cells. Moreover, it enhanced IL-5 and IL-13 production in response to TCR and IL-33 stimulation from a specific subset of pathogenic Th2 cells that expresses a high level of IL-33 receptor, which exacerbated allergic airway inflammation in a mouse model of asthma. Mechanistically, exophilin-5 regulates extracellular superoxide release, intracellular ROS production, and phosphoinositide 3-kinase activity by controlling intracellular trafficking of Nox2-containing vesicles, which seems to prevent the overactivation of pathogenic Th2 cells mediated by IL-33. This is the first report to our knowledge to establish the significance of the Rab27-related protein exophilin-5 in the development of allergic airway inflammation, and provides insights into the pathophysiology of asthma.

Roles and regulation of myosin V interaction with cargo

A major question in cell biology is, how are organelles and large macromolecular complexes transported within a cell? Myosin V molecular motors play critical roles in the distribution of organelles, vesicles, and mRNA. Mis-localization of organelles that depend on myosin V motors underlie diseases in the skin, gut, and brain. Thus, the delivery of organelles to their proper destination is important for animal physiology and cellular function. Cargoes attach to myosin V motors via cargo specific adaptor proteins, which transiently bridge motors to their cargoes. Regulation of these adaptor proteins play key roles in the regulation of cargo transport. Emerging studies reveal that cargo adaptors play additional essential roles in the activation of myosin V, and the regulation of actin filaments. Here, we review how motor-adaptor interactions are controlled to regulate the proper loading and unloading of cargoes, as well as roles of adaptor proteins in the regulation of myosin V activity and the dynamics of actin filaments.

Membrane trafficking in the retinal pigment epithelium at a glance

The retinal pigment epithelium (RPE) is a highly specialised pigmented monolayer sandwiched between the choroid and the photoreceptors in the retina. Key functions of the RPE include transport of nutrients to the neural retina, removal of waste products and water from the retina to the blood, recycling of retinal chromophores, absorption of scattered light and phagocytosis of the tips of the photoreceptor outer segments. These functions place a considerable membrane trafficking burden on the RPE. In this Cell Science at a Glance article and the accompanying poster, we focus on RPE-specific adaptations of trafficking pathways. We outline mechanisms underlying the polarised expression of membrane proteins, melanosome biogenesis and movement, and endocytic trafficking, as well as photoreceptor outer segment phagocytosis and degradation. We also briefly discuss theories of how dysfunction in trafficking pathways contributes to retinal disease.

Rab27a co-ordinates actin-dependent transport by controlling organelle-associated motors and track assembly proteins

Cell biologists generally consider that microtubules and actin play complementary roles in long- and short-distance transport in animal cells. On the contrary, using melanosomes of melanocytes as a model, we recently discovered that the motor protein myosin-Va works with dynamic actin tracks to drive long-range organelle dispersion in opposition to microtubules. This suggests that in animals, as in yeast and plants, myosin/actin can drive long-range transport. Here, we show that the SPIRE-type actin nucleators (predominantly SPIRE1) are Rab27a effectors that co-operate with formin-1 to generate actin tracks required for myosin-Va-dependent transport in melanocytes. Thus, in addition to melanophilin/myosin-Va, Rab27a can recruit SPIREs to melanosomes, thereby integrating motor and track assembly activity at the organelle membrane. Based on this, we suggest a model in which organelles and force generators (motors and track assemblers) are linked, forming an organelle-based, cell-wide network that allows their collective activity to rapidly disperse the population of organelles long-distance throughout the cytoplasm.

The road to lysosome-related organelles: Insights from Hermansky-Pudlak syndrome and other rare diseases

Lysosome-related organelles (LROs) comprise a diverse group of cell type-specific, membrane-bound subcellular organelles that derive at least in part from the endolysosomal system but that have unique contents, morphologies and functions to support specific physiological roles. They include: melanosomes that provide pigment to our eyes and skin; alpha and dense granules in platelets, and lytic granules in cytotoxic T cells and natural killer cells, which release effectors to regulate hemostasis and immunity; and distinct classes of lamellar bodies in lung epithelial cells and keratinocytes that support lung plasticity and skin lubrication. The formation, maturation and/or secretion of subsets of LROs are dysfunctional or entirely absent in a number of hereditary syndromic disorders, including in particular the Hermansky-Pudlak syndromes. This review provides a comprehensive overview of LROs in humans and model organisms and presents our current understanding of how the products of genes that are defective in heritable diseases impact their formation, motility and ultimate secretion.

In Vitro Fluorescence Resonance Energy Transfer-Based Assay Used to Determine the Rab27-Effector-Binding Affinity

The Rab27 subfamily consists of Rab27a/b isoforms that have similar but not identical functions. Those functions include the regulation of trafficking, docking, and fusion of various lysosome-related organelles and secretory granules; such as melanosomes in melanocytes and lytic granules in cytotoxic T lymphocytes. Rab27a/b exert their specific and versatile functions by interacting with 11 effector proteins, preferentially in their GTP-bound state. In recent years, a number of studies have identified roles for Rab27 proteins and their effectors in cancer cell invasion and metastasis, immune response, inflammation, and allergic responses. These findings suggest that Rab27-effector protein interaction inhibitors could contribute to the development of effective strategies to treat these diseases. To facilitate inhibitor identification, in this study we developed a fluorescence resonance energy transfer-based protein-protein interaction assay that reports Rab27-effector interactions. Green fluorescent protein (GFP)-mouse (m) synaptotagmin-like protein (Slp)1 and GFP-mSlp2 (N-terminus Rab27-binding domains) recombinant proteins were used as donor fluorophores, whereas mCherry-human (h) Rab27a/b recombinant proteins were used as acceptor fluorophores. The in vitro binding affinity of mSlp2 to Rab27 was found to be higher compared with mSlp1 and was evidenced by the effective concentration 50 value differences (mSlp2-hRab27b = 0.15 μM < mSlp2-hRab27a = 0.2 μM < mSlp1-hRab27a = 0.32 μM < mSlp1-hRab27b = 0.33 μM). The specificity of the assay was assessed using unlabeled rat (r) Rab27a and hRab27b recombinant proteins as typical competitive inhibitors for Rab27-effector interactions and was evidenced by the inhibitory concentration 50 value differences. Accordingly, this in vitro assay can be employed in identification of candidate inhibitors of Rab27-effector interactions.

Molecular underpinnings of cytoskeletal cross-talk

Cross-talk between the microtubule and actin networks has come under intense scrutiny following the realization that it is crucial for numerous essential processes, ranging from cytokinesis to cell migration. It is becoming increasingly clear that proteins long-considered highly specific for one or the other cytoskeletal system do, in fact, make use of both filament types. How this functional duality of "shared proteins" has evolved and how their coadaptation enables cross-talk at the molecular level remain largely unknown. We previously discovered that the mammalian adaptor protein melanophilin of the actin-associated myosin motor is one such "shared protein," which also interacts with microtubules in vitro. In a hypothesis-driven in vitro and in silico approach, we turn to early and lower vertebrates and ask two fundamental questions. First, is the capability of interacting with microtubules and actin filaments unique to mammalian melanophilin or did it evolve over time? Second, what is the functional consequence of being able to interact with both filament types at the cellular level? We describe the emergence of a protein domain that confers the capability of interacting with both filament types onto melanophilin. Strikingly, our computational modeling demonstrates that the regulatory power of this domain on the microscopic scale alone is sufficient to recapitulate previously observed behavior of pigment organelles in amphibian melanophores. Collectively, our dissection provides a molecular framework for explaining the underpinnings of functional cross-talk and its potential to orchestrate the cell-wide redistribution of organelles on the cytoskeleton.

Knockdown of microRNA‑143‑5p by STTM technology affects eumelanin and pheomelanin production in melanocytes

MicroRNAs (miRNAs) serve various roles in the regulation of melanogenesis in mammalian melanocytes that contribute to the development of hair color. The manipulation of the melanocyte action is a new target for genetic improvement. Short tandem target mimic (STTM) is a potent approach for silencing miRNAs in plants and animals. To investigate the function of miR‑143‑5p in melanogenesis, STTM was used to block the expression of miR‑143‑5p (STTM‑miR‑143‑5p). The molecular analysis and luciferase reporter assay identified myosin Va gene (MYO5A) as one of the miR‑143‑5p targets. STTM‑miR‑143‑5p overexpression resulted in an increased expression of downstream melanogenesis genes including microphthalmia‑associated transcription factor (MITF), tyrosinase family members [tyrosinase (TYR) and tyrosinase‑related protein 1 (TYRP1)], melanophilin (MLPH), and Rab27a, thereby contributing to melanocyte pigmentation by promoting total alkali‑soluble melanogenesis (ASM) and eumelanin (EM) contents; conversely, STTM‑miR‑143‑5p overexpression resulted in decreased expression of the tyrosinase‑related protein 2 (TYRP2)/dopachrome tautomerase (DCT), which is responsible for decreased pheomelanin (PM) content in mouse melanocytes. The results indicated that melanin production in melanocytes could be increased by manipulating miR‑143‑5p expression using STTM which resulted in ASM and EM production.

Noncanonical Hippo Signalling in the Regulation of Leukocyte Function

The mammalian sterile 20-like (MST) kinases are central constituents of the evolutionary ancient canonical Hippo pathway regulating cell proliferation and survival. However, perhaps surprisingly, MST1 deficiency in human patients leads to a severe combined immunodeficiency syndrome with features of autoimmune disease. In line with this, Mst1-deficient mice exhibit severe defects in lymphocyte and neutrophil functions as well as disturbed intracellular vesicle transport. These findings spurred research on the noncanonical functions of MST1 in leukocytes. Here, we summarise the latest findings on this topic and discuss MST1 as a critical regulator of various leukocyte functions.

Regulation of class V myosin

Class V myosin (myosin-5) is a molecular motor that functions as an organelle transporter. The activation of myosin-5's motor function has long been known to be associated with a transition from the folded conformation in the off-state to the extended conformation in the on-state, but only recently have we begun to understand the underlying mechanism. The globular tail domain (GTD) of myosin-5 has been identified as the inhibitory domain and has recently been shown to function as a dimer in regulating the motor function. The folded off-state of myosin-5 is stabilized by multiple intramolecular interactions, including head-GTD interactions, GTD-GTD interactions, and interactions between the GTD and the C-terminus of the first coiled-coil segment. Any cellular factor that affects these intramolecular interactions and thus the stability of the folded conformation of myosin-5 would be expected to regulate myosin-5 motor function. Both the adaptor proteins of myosin-5 and Ca2+ are potential regulators of myosin-5 motor function, because they can destabilize its folded conformation. A combination of these regulators provides a versatile scheme in regulating myosin-5 motor function in the cell.

Myosin Va's adaptor protein melanophilin enforces track selection on the microtubule and actin networks in vitro

Pigment organelles, or melanosomes, are transported by kinesin, dynein, and myosin motors. As such, melanosome transport is an excellent model system to study the functional relationship between the microtubule- and actin-based transport systems. In mammalian melanocytes, it is well known that the Rab27a/melanophilin/myosin Va complex mediates actin-based transport in vivo. However, pathways that regulate the overall directionality of melanosomes on the actin/microtubule networks have not yet been delineated. Here, we investigated the role of PKA-dependent phosphorylation on the activity of the actin-based Rab27a/melanophilin/myosin Va transport complex in vitro. We found that melanophilin, specifically its C-terminal actin-binding domain (ABD), is a target of PKA. Notably, in vitro phosphorylation of the ABD closely recapitulated the previously described in vivo phosphorylation pattern. Unexpectedly, we found that phosphorylation of the ABD affected neither the interaction of the complex with actin nor its movement along actin tracks. Surprisingly, the phosphorylation state of melanophilin was instead important for reversible association with microtubules in vitro. Dephosphorylated melanophilin preferred binding to microtubules even in the presence of actin, whereas phosphorylated melanophilin associated with actin. Indeed, when actin and microtubules were present simultaneously, melanophilin's phosphorylation state enforced track selection of the Rab27a/melanophilin/myosin Va transport complex. Collectively, our results unmasked the regulatory dominance of the melanophilin adaptor protein over its associated motor and offer an unexpected mechanism by which filaments of the cytoskeletal network compete for the moving organelles to accomplish directional transport on the cytoskeleton in vivo.

Exploring the iceberg: Prospects of coordinated myosin V and actin assembly functions in transport processes

Spir actin nucleators and myosin V motor proteins were recently discovered to coexist in a protein complex. The direct interaction allows the coordinated activation of actin motor proteins and actin filament track generation at vesicle membranes. By now the cooperation of myosin V (MyoV) motors and Spir actin nucleation function has only been shown in the exocytic transport of Rab11 vesicles in metaphase mouse oocytes. Next to Rab11, myosin V motors however interact with a variety of Rab GTPases including Rab3, Rab8 and Rab10. As a common theme most of the MyoV interacting Rab GTPases function at different steps along the exocytic transport routes. We here summarize the different transport functions of class V myosins and provide as proof of principle data showing a colocalization of Spir actin nucleators and MyoVa at Rab8a vesicles. This suggests that besides Rab11/MyoV transport also the Rab8/MyoV and possibly other MyoV transport processes recruit Spir actin filament nucleation function.

Two pathways regulate cortical granule translocation to prevent polyspermy in mouse oocytes

An egg must be fertilized by a single sperm only. To prevent polyspermy, the zona pellucida, a structure that surrounds mammalian eggs, becomes impermeable upon fertilization, preventing the entry of further sperm. The structural changes in the zona upon fertilization are driven by the exocytosis of cortical granules. These translocate from the oocyte's centre to the plasma membrane during meiosis. However, very little is known about the mechanism of cortical granule translocation. Here we investigate cortical granule transport and dynamics in live mammalian oocytes by using Rab27a as a marker. We show that two separate mechanisms drive their transport: myosin Va-dependent movement along actin filaments, and an unexpected vesicle hitchhiking mechanism by which cortical granules bind to Rab11a vesicles powered by myosin Vb. Inhibiting cortical granule translocation severely impaired the block to sperm entry, suggesting that translocation defects could contribute to miscarriages that are caused by polyspermy.

Mammalian eggs release cortical granules to avoid being fertilized by more than a single sperm as polyspermy results in nonviable embryos. Here, the authors describe the mechanism driving translocation of the granules to the cortex in the mouse egg and show this process is essential to prevent polyspermy.

Rab3a and Rab10 are regulators of lysosome exocytosis and plasma membrane repair

Disruption of the cell plasma membrane can occur due to mechanical damage, pore forming toxins, etc. Resealing or plasma membrane repair (PMR) is the emergency response required for cell survival. It is triggered by Ca²⁺ entering through the disruption, causing organelles such as lysosomes located underneath the plasma membrane to fuse rapidly with the adjacent plasma membrane. We have recently identified some of the molecular traffic machinery that is involved in this vital process. Specifically, we showed that 2 members of the Rab family of small GTPases, Rab3a and Rab10, are essential for lysosome exocytosis and PMR in cells challenged with a bacterial toxin, streptolysin-O (SLO). Additionally, we showed that Rab3a regulates PMR via the interaction with 2 effectors, synaptotagmin-like protein 4a (Slp4-a) and nonmuscle myosin heavy chain IIA (NMHC IIA), the latter being identified for the first time as a Rab3a effector. This tripartite complex is essential for the positioning of the peripheral lysosomes responsible for PMR. In cells lacking any of the components of this tripartite complex, lysosomes were concentrated in the perinuclear region and absent in the periphery culminating with PMR inhibition.

Analysis of the interactions between Rab GTPases and class V myosins

Myosins are actin-based motor proteins that are involved in a wide variety of cellular processes such as membrane transport, muscle contraction, and cell division. Humans have over 40 myosins that can be placed into 18 classes, the malfunctioning of a number of which can lead to disease. There are three members of the human class V myosin family, myosins Va, Vb, and Vc. People lacking functional myosin Va suffer from a rare autosomal recessive disease called Griscelli's Syndrome type I (GS1) that is characterized by severe neurological defects and partial albinism. Mutations in the myosin Vb gene lead to an epithelial disorder called microvillus inclusion disease (MVID) that is often fatal in infants. The class V myosins have been implicated in the transport of diverse cargoes such as melanosomes in pigment cells, synaptic vesicles in neurons, RNA transcripts in a variety of cell types, and organelles such as the endoplasmic reticulum. The Rab GTPases play a critical role in recruiting class V myosins to their cargo. We recently published a study in which we used the yeast two-hybrid system to systematically test myosin Va for its ability to interact with each member of the human Rab GTPase family. We present here a detailed description of this yeast two-hybrid "living chip" assay. Furthermore, we present a protocol for validating positive interactions obtained from this screen by coimmunoprecipitation.

Myosin Vb mediated plasma membrane homeostasis regulates peridermal cell size and maintains tissue homeostasis in the zebrafish epidermis

The epidermis is a stratified epithelium, which forms a barrier to maintain the internal milieu in metazoans. Being the outermost tissue, growth of the epidermis has to be strictly coordinated with the growth of the embryo. The key parameters that determine tissue growth are cell number and cell size. So far, it has remained unclear how the size of epidermal cells is maintained and whether it contributes towards epidermal homeostasis. We have used genetic analysis in combination with cellular imaging to show that zebrafish goosepimples/myosin Vb regulates plasma membrane homeostasis and is involved in maintenance of cell size in the periderm, the outermost epidermal layer. The decrease in peridermal cell size in Myosin Vb deficient embryos is compensated by an increase in cell number whereas decrease in cell number results in the expansion of peridermal cells, which requires myosin Vb (myoVb) function. Inhibition of cell proliferation as well as cell size expansion results in increased lethality in larval stages suggesting that this two-way compensatory mechanism is essential for growing larvae. Our analyses unravel the importance of Myosin Vb dependent cell size regulation in epidermal homeostasis and demonstrate that the epidermis has the ability to maintain a dynamic balance between cell size and cell number.

Myosin-Va and dynamic actin oppose microtubules to drive long-range organelle transport

In animal cells, microtubule and actin tracks and their associated motors (dynein, kinesin, and myosin) are thought to regulate long- and short-range transport, respectively. Consistent with this, microtubules extend from the perinuclear centrosome to the plasma membrane and allow bidirectional cargo transport over long distances (>1 μm). In contrast, actin often comprises a complex network of short randomly oriented filaments, suggesting that myosin motors move cargo short distances. These observations underpin the "highways and local roads" model for transport along microtubule and actin tracks. The "cooperative capture" model exemplifies this view and suggests that melanosome distribution in melanocyte dendrites is maintained by long-range transport on microtubules followed by actin/myosin-Va-dependent tethering. In this study, we used cell normalization technology to quantitatively examine the contribution of microtubules and actin/myosin-Va to organelle distribution in melanocytes. Surprisingly, our results indicate that microtubules are essential for centripetal, but not centrifugal, transport. Instead, we find that microtubules retard a centrifugal transport process that is dependent on myosin-Va and a population of dynamic F-actin. Functional analysis of mutant proteins indicates that myosin-Va works as a transporter dispersing melanosomes along actin tracks whose +/barbed ends are oriented toward the plasma membrane. Overall, our data highlight the role of myosin-Va and actin in transport, and not tethering, and suggest a new model in which organelle distribution is determined by the balance between microtubule-dependent centripetal and myosin-Va/actin-dependent centrifugal transport. These observations appear to be consistent with evidence coming from other systems showing that actin/myosin networks can drive long-distance organelle transport and positioning.

Human immunodeficiency syndromes affecting human natural killer cell cytolytic activity

Natural killer (NK) cells are lymphocytes of the innate immune system that secrete cytokines upon activation and mediate the killing of tumor cells and virus-infected cells, especially those that escape the adaptive T cell response caused by the down regulation of MHC-I. The induction of cytotoxicity requires that NK cells contact target cells through adhesion receptors, and initiate activation signaling leading to increased adhesion and accumulation of F-actin at the NK cell cytotoxic synapse. Concurrently, lytic granules undergo minus-end directed movement and accumulate at the microtubule-organizing center through the interaction with microtubule motor proteins, followed by polarization of the lethal cargo toward the target cell. Ultimately, myosin-dependent movement of the lytic granules toward the NK cell plasma membrane through F-actin channels, along with soluble N-ethylmaleimide-sensitive factor attachment protein receptor-dependent fusion, promotes the release of the lytic granule contents into the cleft between the NK cell and target cell resulting in target cell killing. Herein, we will discuss several disease-causing mutations in primary immunodeficiency syndromes and how they impact NK cell-mediated killing by disrupting distinct steps of this tightly regulated process.

Evidence for defective Rab GTPase-dependent cargo traffic in immune disorders

A fully functional immune system is essential to protect the body against pathogens and other diseases, including cancer. Vesicular trafficking provides the correct localization of proteins within all cell types, but this process is most exquisitely controlled and coordinated in immune cells because of their specialized organelles and their requirement to respond to selected stimuli. More than 60 Rab GTPases play important roles in protein trafficking, but only five Rab-encoding genes have been associated with inherited human disorders, and only one of these (Rab27a) causes an immune defect. Mutations in RAB27A cause Griscelli Syndrome type 2 (GS2), an autosomal recessive disorder of pigmentation and severe immune deficiency. In lymphocytes, Munc13-4 is an effector of Rab27a, and mutations in the gene encoding this protein (UNC13D) cause Familial Hemophagocytic Lymphohistiocytosis Type 3 (FHL3). The immunological features of GS2 and FHL3 include neutropenia, thrombocytopenia, and immunodeficiency due to impaired function of cytotoxic lymphocytes. The small number of disorders caused by mutations in genes encoding Rabs could be due to their essential functions, where defects in these genes could be lethal. However, with the increasing use of next generation sequencing technologies, more mutations in genes encoding Rabs may be identified in the near future.

Identification and characterization of multiple novel Rab-myosin Va interactions

A systematic screen of the entire human Rab GTPase family for interactions with myosin Va identified 10 novel Rab partners for myosin Va, all of which belong to the endocytic recycling and post-Golgi secretory membrane network. However, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes.

Myosin Va is a widely expressed actin-based motor protein that binds members of the Rab GTPase family (3A, 8A, 10, 11A, 27A) and is implicated in many intracellular trafficking processes. To our knowledge, myosin Va has not been tested in a systematic screen for interactions with the entire Rab GTPase family. To that end, we report a yeast two-hybrid screen of all human Rabs for myosin Va-binding ability and reveal 10 novel interactions (3B, 3C, 3D, 6A, 6A′, 6B, 11B, 14, 25, 39B), which include interactions with three new Rab subfamilies (Rab6, Rab14, Rab39B). Of interest, myosin Va interacts with only a subset of the Rabs associated with the endocytic recycling and post-Golgi secretory systems. We demonstrate that myosin Va has three distinct Rab-binding domains on disparate regions of the motor (central stalk, an alternatively spliced exon, and the globular tail). Although the total pool of myosin Va is shared by several Rabs, Rab10 and Rab11 appear to be the major determinants of its recruitment to intracellular membranes. We also present evidence that myosin Va is necessary for maintaining a peripheral distribution of Rab11- and Rab14-positive endosomes.

Reduced glutathione disrupts the intracellular trafficking of tyrosinase and tyrosinase-related protein-1 but not dopachrome tautomerase and Pmel17 to melanosomes, which results in the attenuation of melanization

We previously reported that treatment of B16 melanotic melanoma cells with reduced glutathione (GSH) converts them to amelanotic cells without any significant down-regulation of tyrosinase activity. To characterize the cellular mechanism(s) involved, we determined the intracellular distribution of melanocyte-specific proteins, especially in melanin synthesis-specific organelles, termed melanosomes by subcellular fractionation followed by Western blotting and confocal laser microscopy (CFLM). In the melanosome-rich large granule fraction and in highly purified melanosome fractions, while GSH-induced amelanotic B16 cells have significantly diminished levels of protein/activity of tyrosinase and tyrosinase-related protein-1 compared with control melanized B16 cells, there was substantially no difference in the distribution and levels of dopachrome tautomerase and the processed isoform of Pmel17 (HMB45) between control melanized and GSH-induced amelanotic B16 cells. Analysis of merged images obtained by CFLM revealed that whereas tyrosinase, Pmel17 and dopachrome tautomerase colocalize with each other in the control melanized B16 cells, tyrosinase does not colocalize with Pmel17 or its processed isoform and with dopachrome tautomerase in GSH-induced amelanotic B16 cells. The sum of these findings suggests that reduced glutathione selectively disrupts the intracellular trafficking of tyrosinase and tyrosinase-related protein-1 but not dopachrome tautomerase and Pmel17 to melanosomes, which results in the attenuation of melanization, probably serving as a putative model for oculocutaneous albinism type 4.

Rab27a and melanosomes: a model to investigate the membrane targeting of Rabs

Rab proteins constitute the largest family within the Ras superfamily of small GTPases (>60 in mammals) and are essential regulators of transport between intracellular organelles. Key to this activity is their targeting to specific compartments within the cell. However, although great strides have been made over the last 25 years in assigning functions to individual Rabs and identifying their downstream effectors, the mechanism(s) regulating their targeting to specific subcellular membranes remains less well understood. In the present paper, we review the evidence supporting the proposed mechanisms of Rab targeting and highlight insights into this process provided by studies of Rab27a.

Regulation of vesicular trafficking and leukocyte function by Rab27 GTPases and their effectors

The Rab27 family of GTPases regulates the efficiency and specificity of exocytosis in hematopoietic cells, including neutrophils, CTLs, NK cells, and mast cells. However, the mechanisms regulated by Rab27 GTPases are cell-specific, as they depend on the differential expression and function of particular effector molecules that are recruited by the GTPases. In addition, Rab27 GTPases participate in multiple steps of the regulation of the secretory process, including priming, tethering, docking, and fusion through sequential interaction with multiple effector molecules. Finally, recent reports suggest that Rab27 GTPases and their effectors regulate vesicular trafficking mechanisms other than exocytosis, including endocytosis and phagocytosis. This review focuses on the latest discoveries on the function of Rab27 GTPases and their effectors Munc13-4 and Slp1 in neutrophil function comparatively to their functions in other leukocytes.

Oligomerization of rab/effector complexes in the regulation of vesicle trafficking

Rabs comprise the largest member of the Ras superfamily of small GTPases with over 60 proteins in mammals and 11 proteins in yeast. Like all small GTPases, Rabs oscillate between an inactive GDP-bound conformation and an active GTP-bound state that is tethered to lipid membranes via a C-terminal prenylation site on conserved cysteine residues. In their active state, Rabs regulate various aspects of membrane trafficking, including vesicle formation, transport, docking, and fusion. The critical element of biological activity is the recruitment of cytosolic effector proteins to specific endomembranes by active Rabs. The importance of Rabs in cellular processes is apparent from their links to genetic disorders, immunodeficiency, cancer, and pathogen invasion. During the last decade, numerous structures of complexes have shed light on the molecular basis for Rab/effector specificity and their topological organization on subcellular membranes. Here, I review the known structures of Rab/effector complexes and their modes of oligomerization. This is followed by a brief discussion on the thermodynamics of effector recruitment, which has not been documented sufficiently in previous reviews. A summary of diseases associated with Rab/effector trafficking pathways concludes this chapter.

Identification of miR-145 as a key regulator of the pigmentary process

The current treatments for hyperpigmentation are often associated with a lack of efficacy and adverse side effects. We hypothesized that microRNA (miRNA)-based treatments may offer an attractive alternative by specifically targeting key genes in melanogenesis. The aim of this study was to identify miRNAs interfering with the pigmentary process and to assess their functional role. miRNA profiling was performed on mouse melanocytes after three consecutive treatments involving forskolin and solar-simulated UV (ssUV) irradiation. Sixteen miRNAs were identified as differentially expressed in treated melan-a cells versus untreated cells. Remarkably, a 15-fold downregulation of miR-145 was detected. Overexpression or downregulation of miR-145 in melan-a cells revealed reduced or increased expression of Sox9, Mitf, Tyr, Trp1, Myo5a, Rab27a, and Fscn1, respectively. Moreover, a luciferase reporter assay demonstrated direct targeting of Myo5a by miR-145 in mouse and human melanocytes. Immunofluorescence tagging of melanosomes in miR-145-transfected human melanocytes displayed perinuclear accumulation of melanosomes with additional hypopigmentation of harvested cell pellets. In conclusion, this study has established an miRNA signature associated with forskolin and ssUV treatment. The significant down- or upregulation of major pigmentation genes, after modulating miR-145 expression, suggests a key role for miR-145 in regulating melanogenesis.

Mechanisms of protein delivery to melanosomes in pigment cells

Vertebrate pigment cells in the eye and skin are useful models for cell types that use specialized endosomal trafficking pathways to partition cargo proteins to unique lysosome-related organelles such as melanosomes. This review describes current models of protein trafficking required for melanosome biogenesis in mammalian melanocytes.

Demonstration of melanosome transfer by a shedding microvesicle mechanism

The process of melanosome transfer has fascinated pigment cell biologists for decades. Whole-organelle transfer is a unique property of melanocytes, suggesting that the biologic underpinnings of the process reflect melanocyte- and keratinocyte-specific proteins and pathways. Although several mechanisms of melanosome transfer are likely to occur in the skin, Ando et al. focused on a new mechanism of melanosome transfer that involves release of melanosome-containing globules, similar to shedding vesicles into the extracellular space, followed by uptake by keratinocytes. This model adds further complexity to the process of melanosome transfer in the skin.

Myosin VI regulates actin dynamics and melanosome biogenesis

Myosin VI has been implicated in various steps of organelle dynamics. However, the molecular mechanism by which this myosin contributes to membrane traffic is poorly understood. Here, we report that myosin VI is associated with a lysosome-related organelle, the melanosome. Using an actin-based motility assay and video microscopy, we observed that myosin VI does not contribute to melanosome movements. Myosin VI expression regulates instead the organization of actin networks in the cytoplasm. Using a cell-free assay, we showed that myosin VI recruited actin at the surface of isolated melanosomes. Myosin VI is involved in the endocytic-recycling pathway, and this pathway contributes to the transport of a melanogenic enzyme to maturing melanosomes. We showed that depletion of myosin VI accumulated a melanogenic enzyme in enlarged melanosomes and increased their melanin content. We confirmed the requirement of myosin VI to regulate melanosome biogenesis by analysing the morphology of melanosomes in choroid cells from of the Snell's waltzer mice that do not express myosin VI. Together, our results provide new evidence that myosin VI regulates the organization of actin dynamics at the surface of a specialized organelle and unravel a novel function of this myosin in regulating the biogenesis of this organelle.

Myosin V transports secretory vesicles via a Rab GTPase cascade and interaction with the exocyst complex

Vesicle transport requires four steps: vesicle formation, movement, tethering, and fusion. In yeast, two Rab GTPases, Ypt31/32, are required for post-Golgi vesicle formation. A third Rab GTPase, Sec4, and the exocyst act in tethering and fusion of these vesicles. Vesicle production is coupled to transport via direct interaction between Ypt31/32 and the yeast myosin V, Myo2. Here we show that Myo2 interacts directly with Sec4 and the exocyst subunit Sec15. Disruption of these interactions results in compromised growth and the accumulation of secretory vesicles. We identified the Sec15-binding region on Myo2 and also identified residues on Sec15 required for interaction with Myo2. That Myo2 interacts with Sec15 uncovers additional roles for the exocyst as an adaptor for molecular motors and implies similar roles for structurally related tethering complexes. Moreover, these studies predict that for many pathways, molecular motors attach to vesicles prior to their formation and remain attached until fusion.

Melanosomes on the move: a model to understand organelle dynamics

Advances in live-cell microscopy have revealed the extraordinarily dynamic nature of intracellular organelles. Moreover, movement appears to be critical in establishing and maintaining intracellular organization and organellar and cellular function. Motility is regulated by the activity of organelle-associated motor proteins, kinesins, dyneins and myosins, which move cargo along polar MT (microtubule) and actin tracks. However, in most instances, the motors that move specific organelles remain mysterious. Over recent years, pigment granules, or melanosomes, within pigment cells have provided an excellent model for understanding the molecular mechanisms by which motor proteins associate with and move intracellular organelles. In the present paper, we discuss recent discoveries that shed light on the mechanisms of melanosome transport and highlight future prospects for the use of pigment cells in unravelling general molecular mechanisms of intracellular transport.

The recycling endosome protein Rab17 regulates melanocytic filopodia formation and melanosome trafficking

Rab GTPases including Rab27a, Rab38 and Rab32 function in melanosome maturation or trafficking in melanocytes. A screen to identify additional Rabs involved in these processes revealed the localization of GFP-Rab17 on recycling endosomes (REs) and melanosomes in melanocytic cells. Rab17 mRNA expression is regulated by microphthalmia transcription factor (MITF), a characteristic of known pigmentation genes. Rab17 siRNA knockdown in melanoma cells quantitatively increased melanosome concentration at the cell periphery. Rab17 knockdown did not inhibit melanosome maturation nor movement, but it caused accumulation of melanin inside cells. Double knockdown of Rab17 and Rab27a indicated that Rab17 acts on melanosomes downstream of Rab27a. Filopodia are known to play a role in melanosome transfer, and in Rab17 knockdown cells filopodia formation was inhibited. Furthermore, we show that stimulation of melanoma cells with α-melanocyte-stimulating hormone induces filopodia formation, supporting a role for filopodia in melanosome release. Cell stimulation also caused redistribution of REs to the periphery, and knockdown of additional RE-associated Rabs 11a and 11b produced a similar accumulation of melanosomes and melanin to that seen after loss of Rab17. Our findings reveal new functions for RE and Rab17 in pigmentation through a distal step in the process of melanosome release via filopodia.

Loss of Rab27 function results in abnormal lung epithelium structure in mice

Rab27 small GTPases regulate secretion and movement of lysosome-related organelles such as T cell cytolytic granules and platelet-dense granules. Previous studies indicated that Rab27a and Rab27b are expressed in the murine lung suggesting that they regulate secretory processes in the lung. Consistent with those studies, we found that Rab27a and Rab27b are expressed in cell types that contain secretory granules: alveolar epithelial type II (AEII) and Clara cells. We then used Rab27a/Rab27b double knockout (DKO) mice to examine the functional consequence of loss of Rab27 proteins in the murine lung. Light and electron microscopy revealed a number of morphological changes in lungs from DKO mice when compared with those in control animals. In aged DKO mice we observed atrophy of the bronchiolar and alveolar epithelium with reduction of cells numbers, thinning of the bronchiolar epithelium and alveolar walls, and enlargement of alveolar airspaces. In these samples we also observed increased numbers of activated foamy alveolar macrophages and granulocyte containing infiltrates together with reduction in the numbers of Clara cells and AEII cells compared with control. At the ultrastructural level we observed accumulation of cytoplasmic membranes and vesicles in Clara cells. Meanwhile, AEII cells in DKO accumulated large mature lamellar bodies and lacked immature/precursor lamellar bodies. We hypothesize that the morphological changes observed at the ultrastructural level in DKO samples result from secretory defects in AEII and Clara cells and that over time these defects lead to atrophy of the epithelium.

Clinical presentation of Griscelli syndrome type 2 and spectrum of RAB27A mutations

BACKGROUND

Griscelli syndrome type 2 (GS2) is an autosomal-recessive immunodeficiency caused by mutations in RAB27A, clinically characterized by partial albinism and haemophagocytic lymphohistocytosis (HLH). We evaluated the frequency of RAB27A mutations in 21 unrelated patients with haemophagocytic syndromes without mutations in familial HLH (FHL) causing genes or an established diagnosis of GS2. In addition, we report three patients with known GS2. Moreover, neurological involvement and RAB27A mutations in previously published patients with genetically verified GS2 are reviewed.

PROCEDURE

Mutation analysis of RAB27A was performed by direct DNA sequencing. NK cell activity was evaluated and microscopy of the hair was performed to confirm the diagnosis.

RESULTS

RAB27A mutations were found in 1 of the 21 families. This Swedish family had three affected children with heterozygous compound mutations consisting of a novel splice error mutation, [c.239G>C], and a nonsense mutation, [c.550C>T], p.R184X. The three additional children all carried homozygous RAB27A mutations, one of which is a novel splice error mutation, [c.240-2A>C]. Of note, five of the six patients displayed neurological symptoms, while three out of six patients displayed NK cell activity within normal reference values, albeit low. A literature review revealed that 67% of GS2 patients have been reported with neurological manifestations.

CONCLUSIONS

Identification of RAB27A mutations can facilitate prompt diagnosis and treatment, and aid genetic counselling and prenatal diagnosis. Since five of six patients studied herein initially were diagnosed as having FHL, we conclude that the diagnosis of GS2 may be overlooked, particularly in fair-haired patients with haemophagocytic syndromes.

Myosin Vb localises to nucleoli and associates with the RNA polymerase I transcription complex

It is becoming increasingly clear that the mammalian class V myosins are involved in a wide range of cellular processes such as receptor trafficking, mRNA transport, myelination in oligodendrocytes and cell division. Using paralog-specific antibodies, we observed significant nuclear localisation for both myosin Va and myosin Vb. Myosin Vb was present in nucleoli where it co-localises with RNA polymerase I, and newly synthesised ribosomal RNA (rRNA), indicating that it may play a role in transcription. Indeed, its nucleolar pattern was altered upon treatment with RNA polymerase I inhibitors. In contrast, myosin Va is largely excluded from nucleoli and is unaffected by these inhibitors. Myosin Vb was also found to physically associate with RNA polymerase I and actin in co-immunoprecipitation experiments. We propose that myosin Vb serves a role in rRNA transcription.

Assembly of the biogenesis of lysosome-related organelles complex-3 (BLOC-3) and its interaction with Rab9

The Hermansky-Pudlak syndrome (HPS) is a genetic hypopigmentation and bleeding disorder caused by defective biogenesis of lysosome-related organelles (LROs) such as melanosomes and platelet dense bodies. HPS arises from mutations in any of 8 genes in humans and 16 genes in mice. Two of these genes, HPS1 and HPS4, encode components of the biogenesis of lysosome-related organelles complex-3 (BLOC-3). Herein we show that recombinant HPS1-HPS4 produced in insect cells can be efficiently isolated as a 1:1 heterodimer. Analytical ultracentrifugation reveals that this complex has a molecular mass of 146 kDa, equivalent to that of the native complex and to the sum of the predicted molecular masses of HPS1 and HPS4. This indicates that HPS1 and HPS4 interact directly in the absence of any other protein as part of BLOC-3. Limited proteolysis and deletion analyses show that both subunits interact with one another throughout most of their lengths with the sole exception of a long, unstructured loop in the central part of HPS4. An interaction screen reveals a specific and strong interaction of BLOC-3 with the GTP-bound form of the endosomal GTPase, Rab9. This interaction is mediated by HPS4 and the switch I and II regions of Rab9. These characteristics indicate that BLOC-3 might function as a Rab9 effector in the biogenesis of LROs.

The Rip11/Rab11-FIP5 and kinesin II complex regulates endocytic protein recycling

Sorting and recycling of endocytosed proteins are required for proper cellular function and growth. Internalized receptors either follow a fast constitutive recycling pathway, returning to the cell surface directly from the early endosomes, or a slow pathway that involves transport via perinuclear recycling endosomes. Slow recycling pathways are thought to play a key role in directing recycling proteins to specific locations on cell surfaces, such as the leading edges of motile cells. These pathways are regulated by various Rab GTPases, such as Rab4 and Rab11. Here we characterize the role of Rip11/FIP5, a known Rab11-binding protein, in regulating endocytic recycling. We use a combination of electron and fluorescent microscopy with siRNA-based protein knockdown to show that Rip11/FIP5 is present at the peripheral endosomes, where it regulates the sorting of internalized receptors to a slow recycling pathway. We also identify kinesin II as a Rip11/FIP5-binding protein and show that it is required for directing endocytosed proteins into the same recycling pathway. Thus, we propose that the Rip11/FIP5-kinesin-II complex has a key role in the routing of internalized receptors through the perinuclear recycling endosomes.

Intracellular Transport and Kinesin Superfamily Proteins, KIFs: Structure, Function, and Dynamics

Various molecular cell biology and molecular genetic approaches have indicated significant roles for kinesin superfamily proteins (KIFs) in intracellular transport and have shown that they are critical for cellular morphogenesis, functioning, and survival. KIFs not only transport various membrane organelles, protein complexes, and mRNAs for the maintenance of basic cellular activity, but also play significant roles for various mechanisms fundamental for life, such as brain wiring, higher brain functions such as memory and learning and activity-dependent neuronal survival during brain development, and for the determination of important developmental processes such as left-right asymmetry formation and suppression of tumorigenesis. Accumulating data have revealed a molecular mechanism of cargo recognition involving scaffolding or adaptor protein complexes. Intramolecular folding and phosphorylation also regulate the binding activity of motor proteins. New techniques using molecular biophysics, cryoelectron microscopy, and X-ray crystallography have detected structural changes in motor proteins, synchronized with ATP hydrolysis cycles, leading to the development of independent models of monomer and dimer motors for processive movement along microtubules.

Myosin-Va mediates RNA distribution in primary fibroblasts from multiple organs

Myosin-Va has been shown to have multiple functions in a variety of cell types, including a role in RNA transport in neurons. Using primary cultures of cells from organs of young dilute-lethal (Myo5a(d-l)/Myo5a(d-l)) null mutant mice and wild-type controls, we show that in some, but not all, tissues, RNA distribution is dramatically different in the homozygous null mutant cells. The dependence of RNA localization on myosin-Va correlates with the relative abundance of the brain-specific splicing pattern of the myosin-Va tail. We also show that myosin-Va is involved in RNA localization soon after synthesis, because the effects of its absence are diminished for RNAs that are more than 30 min old. Finally, we show that localization of beta-actin mRNA is significantly changed by the absence of myosin-Va. These results suggest that myosin-Va is involved in a transient transport or tethering function in the perinuclear region.