MyRIP, a novel Rab effector, enables myosin VIIa recruitment to retinal melanosomes

Defects of the myosin VIIa motor protein cause deafness and retinal anomalies in humans and mice. We report on the identification of a novel myosin-VIIa-interacting protein that we have named MyRIP (myosin-VIIa- and Rab-interacting protein), since it also binds to Rab27A in a GTP-dependent manner. In the retinal pigment epithelium cells, MyRIP, myosin VIIa and Rab27A are associated with melanosomes. In transfected PC12 cells, overexpression of MyRIP was shown to interfere with the myosin VIIa tail localization. We propose that a molecular complex composed of Rab27A, MyRIP and myosin VIIa bridges retinal melanosomes to the actin cytoskeleton and thereby mediates the local trafficking of these organelles. The defect of this molecular complex is likely to account for the perinuclear mislocalization of the melanosomes observed in the retinal pigment epithelium cells of myosinVIIa-defective mice.     

The melanosome as a model to study organelle motility in mammals

Melanosomes are lysosome-related organelles within which melanin pigment is synthesized. The molecular motors that allow these organelles to move within melanocytes have been the subject of intense study in several organisms. In mammals, melanosomes travel bi-directionally along microtubule tracks. The anterograde movement, i.e., towards microtubule plus-ends at the periphery, is accomplished by proteins of the kinesin superfamily, whereas the retrograde movement, i.e., towards microtubule minus-ends at the cell center, is achieved by dynein and dynein-associated proteins. At the periphery, melanosomes interact with the actin cytoskeleton via a tripartite complex formed by the small GTPase Rab27a, melanophilin and myosin Va, an actin-based motor. This interaction is essential for the maintenance of a dispersed state of the melanosomes, as shown by the perinuclear clustering of organelles in mutants in any of the referred proteins. In the retinal pigment epithelium, a similar complex formed by Rab27a, a melanophilin homolog called MyRIP and myosin VIIa is probably responsible for the tethering of melanosomes to the actin cytoskeleton. The coordination of motor activities is still poorly characterized, although some models have emerged in recent years and are discussed here. Unraveling regulatory mechanisms responsible for melanosome motility in pigmented cells will provide general insights into organelles dynamics within eukaryotic cells.     

Rab27A and its effector MyRIP link secretory granules to F-actin and control their motion towards release sites

The GTPase Rab27A interacts with myosin-VIIa and myosin-Va via MyRIP or melanophilin and mediates melanosome binding to actin. Here we show that Rab27A and MyRIP are associated with secretory granules (SGs) in adrenal chromaffin cells and PC12 cells. Overexpression of Rab27A, GTPase-deficient Rab27A-Q78L, or MyRIP reduced secretory responses of PC12 cells. Amperometric recordings of single adrenal chromaffin cells revealed that Rab27A-Q78L and MyRIP reduced the sustained component of release. Moreover, these effects on secretion were partly suppressed by the actin-depolymerizing drug latrunculin but strengthened by jasplakinolide, which stabilizes the actin cortex. Finally, MyRIP and Rab27A-Q78L restricted the motion of SGs in the subplasmalemmal region of PC12 cells, as measured by evanescent-wave fluorescence microscopy. In contrast, the Rab27A-binding domain of MyRIP and a MyRIP construct that interacts with myosin-Va but not with actin increased the mobility of SGs. We propose that Rab27A and MyRIP link SGs to F-actin and control their motion toward release sites through the actin cortex.     

Roles of Myosin Va and Rab3D in Membrane Remodeling of Immature Secretory Granules

Neuroendocrine secretory granules (SGs) are formed at the trans-Golgi network (TGN) as immature intermediates. In PC12 cells, these immature SGs (ISGs) are transported within seconds to the cell cortex, where they move along actin filaments and complete maturation. This maturation process comprises acidification-dependent processing of cargo proteins, condensation of the SG matrix, and removal of membrane and proteins not destined to mature SGs (MSGs) into ISG-derived vesicles (IDVs). We investigated the roles of myosin Va and Rab3 isoforms in the maturation of ISGs in neuroendocrine PC12 cells. The expression of dominant-negative mutants of myosin Va or Rab3D blocked the removal of the endoprotease furin from ISGs. Furthermore, expression of mutant Rab3D, but not of mutant myosin Va, impaired cargo processing of SGs. In conclusion, our data suggest an implication of myosin Va and Rab3D in the maturation of SGs where they participate in overlapping but not identical tasks.     

Involvement of the Rab27 binding protein Slac2c/MyRIP in insulin exocytosis

Rab27a is a GTPase associated with insulin-containing secretory granules of pancreatic beta-cells. Selective reduction of Rab27a expression by RNA interference did not alter granule distribution and basal secretion but impaired exocytosis triggered by insulin secretagogues. Screening for potential effectors of the GTPase revealed that the Rab27a-binding protein Slac2c/MyRIP is associated with secretory granules of beta-cells. Attenuation of Slac2c/MyRIP expression by RNA interference did not modify basal secretion but severely impaired hormone release in response to secretagogues. Although beta-cells express Myosin-Va, a potential partner of Slac2c/MyRIP, no functional link between the two proteins could be demonstrated. In fact, overexpression of the Myosin-Va binding domain of Slac2c/MyRIP did not affect granule localization and hormone exocytosis. In contrast, overexpression of the actin-binding domain of Slac2c/MyRIP led to a potent inhibition of exocytosis without detectable alteration in granule distribution. This effect was prevented by point mutations that abolish actin binding. Taken together our data suggest that Rab27a and Slac2c/MyRIP are part of a complex mediating the interaction of secretory granules with cortical actin cytoskeleton and participate to the regulation of the final steps of insulin exocytosis.     

Functional analysis of Slac2-c/MyRIP as a linker protein between melanosomes and myosin VIIa

Slac2-c/MyRIP, an in vitro Rab27A- and myosin Va/VIIa-binding protein, has recently been proposed to regulate retinal melanosome transport in retinal pigment epithelium cells by directly linking melanosome-bound Rab27A and myosin VIIa; however, the exact function of Slac2-c in melanosome transport has never been clarified. In this study, we used melanosome transport in skin melanocytes as a model for retinal melanosome transport and analyzed the in vivo function of Slac2-c in melanosome transport by the ectopic expression of Slac2-c, together with myosin VIIa, in Slac2-a-depleted melanocytes. In vitro binding experiments revealed that myosin VIIa had a greater affinity for Slac2-c, compared with the binding affinity of myosin Va, and that the myosin VIIa-binding domain of Slac2-c is different from the previously characterized myosin Va-binding domain that is conserved between Slac2-a/melanophilin and Slac2-c. Consistent with this result, cyan fluorescent protein-tagged Slac2-c expressed in melanocytes was localized on melanosomes via the specific interaction with Rab27A and recruited co-expressed yellow fluorescent protein-tagged myosin VIIa to the melanosomes without interfering with the normal peripheral melanosome distribution, whereas when myosin VIIa alone was expressed in melanocytes, it was not localized on the melanosomes. Moreover, Slac2-c ectopically expressed in melanocytes did not rescue the perinuclear aggregation phenotype induced by the knockdown of endogenous Slac2-a with a specific small interfering RNA, whereas the expression of the Slac2-c x myosin VIIa complex supported the normal melanosome distribution in Slac2-a-depleted melanocytes, indicating that Slac2-c functions as a myosin VIIa receptor rather than a myosin Va receptor in melanosome transport. Based on these findings, we propose that Slac2-c acts as a functional myosin VIIa receptor and that the Rab27A.Slac2-c x myosin VIIa tripartite protein complex regulates the transport of retinal melanosomes in pigment epithelium cells.     

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

The function of lysosome-related organelles such as melanosomes in melanocytes, and lytic granules in cytotoxic T lymphocytes is disrupted in Griscelli syndrome and related diseases. Griscelli syndrome results from loss of function mutations in either the RAB27A (type 1 Griscelli syndrome) or MYO5A (type 2 Griscelli syndrome) genes. Melanocytes from Griscelli syndrome patients and respective murine models ashen (Rab27a mutant), dilute (myosin Va mutant), and leaden exhibit perinuclear clustering of melanosomes. Recent work suggests that Rab27a is required to recruit myosin Va to melanosomes, thereby tethering melanosomes to the peripheral actin network and promoting melanosome retention at the tips of melanocytic dendrites. Here, we characterize the function of the leaden gene product. We show that Rab27a, but not myosin Va, can be localized to melanosomes in leaden melanocytes, suggesting that the leaden gene product acts downstream of, or in parallel to, Rab27a in melanocytes to promote recruitment of myosin Va to melanosomes. We also observed reduced levels of myosin Va protein in leaden and ashen melanocytes, suggesting that myosin Va stability is influenced by the leaden and ashen gene products. In leaden cytotoxic T lymphocytes, we observed that lytic granules polarize towards the immunological synapse and kill target cells normally. However, in contrast to melanocytes, we found that neither the leaden gene product (melanophilin) nor myosin Va was detectable in cytotoxic T lymphocytes. These results suggest that Rab27a interacts with different classes of effector proteins in melanocytes and cytotoxic T lymphocytes.     

Exophilin8 transiently clusters insulin granules at the actin-rich cell cortex prior to exocytosis

Exophilin8/MyRIP/Slac2-c is an effector protein of the small GTPase Rab27a and is specifically localized on retinal melanosomes and secretory granules. We investigated the role of exophilin8 in insulin granule trafficking. Exogenous expression of exophilin8 in pancreatic β cells or their cell line, MIN6, polarized (exophilin8-positive) insulin granules at the cell corners, where both cortical actin and the microtubule plus-end-binding protein, EB1, were present. Mutation analyses indicated that the ability of exophilin8 to act as a linker between Rab27a and myosin Va is essential for its granule-clustering activity. Moreover, exophilin8 and exophilin8-associated insulin granules were markedly stable and immobile. Total internal reflection fluorescence microscopy indicated that exophilin8 restricts the motion of insulin granules at a region deeper than that where another Rab27a effector, granuphilin, accumulates docked granules directly attached to the plasma membrane. However, the exophilin8-induced immobility of insulin granules was eliminated upon secretagogue stimulation and did not inhibit evoked exocytosis. Furthermore, exophilin8 depletion prevents insulin granules from being transported close to the plasma membrane and inhibits their fusion. These findings indicate that exophilin8 transiently traps insulin granules into the cortical actin network close to the microtubule plus-ends and supplies them for release during the stimulation.     

The actin-binding domain of Slac2-a/melanophilin is required for melanosome distribution in melanocytes

Melanosomes containing melanin pigments are transported from the cell body of melanocytes to the tips of their dendrites by a combination of microtubule- and actin-dependent machinery. Three proteins, Rab27A, myosin Va, and Slac2-a/melanophilin (a linker protein between Rab27A and myosin Va), are known to be essential for proper actin-based melanosome transport in melanocytes. Although Slac2-a directly interacts with Rab27A and myosin Va via its N-terminal region (amino acids 1 to 146) and the middle region (amino acids 241 to 405), respectively, the functional importance of the putative actin-binding domain of the Slac2-a C terminus (amino acids 401 to 590) in melanosome transport has never been elucidated. In this study we showed that formation of a tripartite protein complex between Rab27A, Slac2-a, and myosin Va alone is insufficient for peripheral distribution of melanosomes in melanocytes and that the C-terminal actin-binding domain of Slac2-a is also required for proper melanosome transport. When a Slac2-a deletion mutant (DeltaABD) or point mutant (KA) that lacks actin-binding ability was expressed in melanocytes, the Slac2-a mutants induced melanosome accumulation in the perinuclear region, possibly by a dominant negative effect, the same as the Rab27A-binding-defective mutant of Slac2-a or the myosin Va-binding-defective mutant. Our findings indicate that Slac2-a organizes actin-based melanosome transport in cooperation with Rab27A, myosin Va, and actin.     

Weibel-Palade小体形成和功能研究进展

Weibel-Palade小体(Weibel-Palade body, WPB)是血管内皮细胞一种特殊的分泌性杆状细胞器, 含有多种生物活性分子, 受到刺激后可以非常迅速的释放这些内容物, 参与止血、炎症和血管生成等生理功能。血管假性血友病因子(von Willebrand Factor, vWF)作为该细胞器的主要组成分子, 其多聚体以管状形式在WPB中规则排列, 促使WPB独特的棒状形态的形成。伴随WPB的形成过程, 其他不同的分子如P-选择素、CD63、Rab27A、Rab3D等相继被运送到WPB中发挥其重要的功能。文章就近年来有关WPB形成的机制和功能等方面的进展进行了讨论。     

Slac2-a/melanophilin contains multiple PEST-like sequences that are highly sensitive to proteolysis

The synaptotagmin-like protein homologue lacking C2 domains-a (Slac2-a)/melanophilin was recently identified as the "missing link" between the small GTPase Rab27A and the actin-based motor protein myosin Va. Although formation of a tripartite protein complex by three molecules had been shown to be required for proper melanosome distribution in melanocytes (Kuroda, T. S., Ariga, H., and Fukuda, M. (2003) Mol. Cell. Biol. 23, 5245-5255), the regulatory mechanisms of the complex (i.e. assembly and disassembly of the complex) had never been elucidated. In this study, we discovered that Slac2-a and a closely related isoform, Slac2-c/MyRIP, contain multiple PEST-like sequences (potential signals for rapid protein degradation) in the myosin Va- and actin-binding domains at the C terminus. We found that the C-terminal domain of Slac2-a is highly sensitive to low concentrations of proteases, such as trypsin and calpain, in vitro, whereas the N-terminal Rab27A-binding domain is highly resistant to these proteases. We further found that endogenous calpains selectively cleave Slac2-a, but not Rab27A or myosin Va, in melanocytes. A mutant Slac2-a lacking one of the PEST-like sequences located at the interface between the myosin Va- and actin-binding domains (DeltaPEST; amino acids 399-405) is more stable than the wild-type protein, both in vitro and in melanocytes. Expression of the mutant Slac2-a-DeltaPEST with an N-terminal green fluorescence protein tag often induced perinuclear aggregation of melanosomes ( approximately 40% of the transfected cells) compared with the wild-type Slac2-a. Our findings suggest that protein degradation of Slac2-a is an essential process for proper melanosome distribution in melanocytes.     

The role of Rab27a in the regulation of melanosome distribution within retinal pigment epithelial cells

Melanosomes within the retinal pigment epithelium (RPE) of mammals have long been thought to exhibit no movement in response to light, unlike fish and amphibian RPE. Here we show that the distribution of melanosomes within the mouse RPE undergoes modest but significant changes with the light cycle. Two hours after light onset, there is a threefold increase in the number of melanosomes in the apical processes that surround adjacent photoreceptors. In skin melanocytes, melanosomes are motile and evenly distributed throughout the cell periphery. This distribution is due to the interaction with the cortical actin cytoskeleton mediated by a tripartite complex of Rab27a, melanophilin, and myosin Va. In ashen (Rab27a null) mice RPE, melanosomes are unable to move beyond the adherens junction axis and do not enter apical processes, suggesting that Rab27a regulates melanosome distribution in the RPE. Unlike skin melanocytes, the effects of Rab27a are mediated through myosin VIIa in the RPE, as evidenced by the similar melanosome distribution phenotype observed in shaker-1 mice, defective in myosin VIIa. Rab27a and myosin VIIa are likely to be required for association with and movement through the apical actin cytoskeleton, which is a prerequisite for entry into the apical processes.     

Slac2-c (synaptotagmin-like protein homologue lacking C2 domains-c), a novel linker protein that interacts with Rab27, myosin Va/VIIa,and actin

Slac2-a (synaptotagmin-like protein (Slp) homologue lacking C2 domains-a)/melanophilin is a melanosome-associated protein that links Rab27A on melanosomes with myosin Va, an actin-based motor protein, and formation of the tripartite protein complex (Rab27A.Slac2-a.myosin Va) has been suggested to regulate melanosome transport (Fukuda, M., Kuroda, T. S., and Mikoshiba, K. (2002) J. Biol. Chem. 277, 12432-12436). Here we report the structure of a novel form of Slac2, named Slac2-c, that is homologous to Slac2-a. Slac2-a and Slac2-c exhibit the same overall structure, consisting of a highly conserved N-terminal Slp homology domain (about 50% identity) and a less conserved C-terminal myosin Va-binding domain (about 20% identity). As with other Slac2 members and the Slp family, the Slp homology domain of Slac2-c was found to interact specifically with the GTP-bound form of Rab27A/B both in vitro and in intact cells, and the C-terminal domain of Slac2-c interacted with myosin Va and myosin VIIa. In addition, we discovered that the most C-terminal conserved region of Slac2-a (amino acids 400-590) and Slac2-c (amino acids 670-856), which is not essential for myosin Va binding, directly binds actin and that expression of these regions in PC12 cells and melanoma cells colocalized with actin filaments at the cell periphery, suggesting a novel role of Slac2-a/c in capture of Rab27-containing organelles in the actin-enriched cell periphery.     

Rab GTPases and myosin motors in organelle motility

The actin cytoskeleton is essential to ensure the proper location of, and communication between, intracellular organelles. Some actin-based myosin motors have been implicated in this process, particularly members of the class V myosins. We discuss here the emerging role of the Ras-like GTPases of the Rab family as regulators of myosin function in organelle transport. Evidence from yeast secretory vesicles and mitochondria, and mammalian melanosomes and endosomes suggests that Rab GTPases are crucial components of the myosin organelle receptor machinery. Better understood is the case of the melanosome where Rab27a recruits a specific effector called melanophilin, which in turn binds myosin Va. The presence of a linker protein between a Rab and a myosin may represent a general mechanism. We argue that Rabs are ideally suited to perform this role as they are exquisite organelle markers. Furthermore, the molecular switch property of Rabs may enable them to regulate the timing of the myosin association with the target organelle.     

A family of Rab27-binding proteins. Melanophilin links Rab27a and myosin Va function in melanosome transport

The Rab27a GTPase regulates diverse processes involving lysosome-related organelles, including melanosome motility in melanocytes, and lytic granule release in cytotoxic T lymphocytes. Toward an understanding of Rab27a function, we searched for proteins that interact with Rab27a(GTP) using the yeast two-hybrid system and identified JFC1/Slp1, a protein of unknown function. JFC1/Slp1 and related proteins, including melanophilin, contain a conserved amino-terminal domain similar to the Rab3a-binding domain of Rabphilin-3. We used several methods to demonstrate that this conserved amino-terminal domain is a Rab27-binding domain. We show that this domain interacts directly, and in a GTP-dependent manner with Rab27a. Furthermore, overexpression of this domain in melanocytes results in perinuclear clustering of melanosomes, suggesting that this region is sufficient for interaction with, and perturbation of function of, Rab27a in a physiological context. Thus, we identified a novel family of Rab27-binding proteins. We also show that melanophilin associates with Rab27a and myosin Va on melanosomes in melanocytes, and present evidence that a domain within the carboxyl-terminal region of melanophilin interacts with the carboxyl-terminal tail of the melanocyte-specific splice isoform of myosin Va. Thus, melanophilin can associate simultaneously with activated Rab27a and myosin Va via distinct regions, and serve as a linker between these proteins.     

Activation of myosin Va function by melanophilin, a specific docking partner of myosin Va

It is known that melanophilin is a myosin Va-targeting molecule that links myosin Va and the cargo vesicles in cells. Here we found that melanophilin directly activates the actin-activated ATPase activity of myosin Va and thus its motor activity. The actin-activated ATPase activity of the melanocyte-type myosin Va having exon-F was significantly activated by melanophilin by 4-fold. Although Rab27a binds to myosin Va/melanophilin complex, it did not affect the melanophilin-induced activation of myosin Va. Deletion of the C-terminal actin binding domain and N-terminal Rab binding domain of melanophilin resulted in no change in the activation of the ATPase by melanophilin, indicating that the myosin Va binding domain (MBD) is sufficient for the activation of myosin Va. Among MBDs, the interaction of MBD-2 with exon-F of myosin Va is critical for the binding of myosin Va and melanophilin, whereas MBD-1 interacting with the globular tail of myosin Va plays a more significant role in the activation of myosin Va ATPase activity. This is the first demonstration that the binding of the cargo molecule directly activates myosin motor activity. The present finding raises the idea that myosin motors are switched upon their binding to the cargo molecules, thus avoiding the waste of ATP consumption.     

Distinct Roles of Myosin Va in Membrane Remodeling and Exocytosis of Secretory Granules

Hormone‐ and neuropeptide‐containing secretory granules (SGs) of neuroendocrine PC12 cells are formed at the trans‐ Golgi network as immature SGs. These intermediates are converted to mature SGs in a complex maturation process, including matrix condensation, processing of cargo proteins and removal of proteins and membrane in clathrin‐coated vesicles. The resulting mature SGs undergo Ca²⁺‐dependent exocytosis upon an appropriate stimulus. We here show that the motor protein myosin Va is implicated in a maturation step of SGs, their binding to F‐actin and their stimulated exocytosis. Interference with myosin Va function blocked the removal of the transmembrane protein furin from maturing SGs without affecting condensation and processing of proteins of the SG lumen. Furthermore, the ATP‐inhibited binding of SGs to F‐actin decreased with progressive maturation and upon interference with myosin Va function. Moreover, the expression of a dominant‐negative myosin Va‐tail or shRNA‐based downregulation of myosin Va interfered with stimulated exocytosis of SGs. In summary, our data suggest an essential function of myosin Va in the membrane remodeling of SGs during maturation and a role in their exocytosis.     

Secretion Pores in Human Endothelial Cells during Acute Hypoxia

Weibel-Palade bodies (WPB) are endothelial vesicles that store von Willebrand factor (vWF), involved in the early phase of hemostasis. In the present study we investigated the morphodynamics of single WPB plasma membrane fusion events upon hypoxic stimulation by using atomic force microscopy (AFM). Simultaneously, we measured vWF release from endothelial cells to functionally confirm WPB exocytosis. Exposing human endothelial cells to hypoxia (pO2 = 5 mm Hg) we found an acute (within minutes) release of vWF. Despite acute vWF release, potential cellular modulators of secretion, such as intracellular pH and cell volume, remained unchanged. We only detected a slight instantaneous increase of cytosolic Ca2+ concentration. Although overall cell morphology remained virtually unchanged, high resolution AFM images of hypoxic endothelial cells disclosed secretion pores, most likely the loci of WPB exocytosis on luminal plasma membrane. We conclude that short-term hypoxia barely alters overall cell morphology and intracellular milieu. However, at nanometer scale, hypoxia instantaneously switches the smooth luminal plasma membrane to a rough activated cell surface, covered with secretion pores that release vWF to the luminal cell surface.     

Rab27a is an essential component of melanosome receptor for myosin Va

Melanocytes that lack the GTPase Rab27a (ashen) are disabled in myosin Va-dependent melanosome capture because the association of the myosin with the melanosome surface depends on the presence of this resident melanosomal membrane protein. One interpretation of these observations is that Rab27a functions wholly or in part as the melanosome receptor for myosin Va (Myo5a). Herein, we show that the ability of the myosin Va tail domain to localize to the melanosome and generate a myosin Va null (dilute) phenotype in wild-type melanocytes is absolutely dependent on the presence of exon F, one of two alternatively spliced exons present in the tail of the melanocyte-spliced isoform of myosin Va but not the brain-spliced isoform. Exon D, the other melanocyte-specific tail exon, is not required. Similarly, the ability of full-length myosin Va to colocalize with melanosomes and to rescue their distribution in dilute melanocytes requires exon F but not exon D. These results predict that an interaction between myosin Va and Rab27a should be exon F dependent. Consistent with this, Rab27a present in detergent lysates of melanocytes binds to beads coated with purified, full-length melanocyte myosin Va and melanocyte myosin Va lacking exon D, but not to beads coated with melanocyte myosin Va lacking exon F or brain myosin Va. Moreover, the preparation of melanocyte lysates in the presence of GDP rather than guanosine-5'-O-(3-thio)triphosphate reduces the amount of Rab27a bound to melanocyte myosin Va-coated beads by approximately fourfold. Finally, pure Rab27a does not bind to myosin Va-coated beads, suggesting that these two proteins interact indirectly. Together, these results argue that Rab27a is an essential component of a protein complex that serves as the melanosome receptor for myosin Va, suggest that this complex contains at least one additional protein capable of bridging the indirect interaction between Rab27a and myosin Va, and imply that the recruitment of myosin Va to the melanosome surface in vivo should be regulated by factors controlling the nucleotide state of Rab27a.     

Slac2-a/melanophilin, the missing link between Rab27 and myosin Va: implications of a tripartite protein complex for melanosome transport

Myosin Va is a member of the unconventional class V myosin family, and a mutation in the myosin Va gene causes pigment granule transport defects in human Griscelli syndrome and dilute mice. How myosin Va recognizes its cargo (i.e. melanosomes), however, has remained undetermined over the past decade. In this study, we discovered Slac2-a/melanophilin to be the "missing link" between myosin Va and GTP-Rab27A present in the melanosome. Deletion analysis and site-directed mutagenesis showed that the N-terminal Slp (synaptotagmin-like protein) homology domain of Slac2-a specifically binds Rab27A/B isoforms and that the C-terminal half directly binds the globular tail of myosin Va. The tripartite protein complex (Rab27A.Slac2-a.myosin Va) in melanoma cells was further confirmed by immunoprecipitation. The discovery that myosin Va indirectly recognizes its cargo through Slac2-a, a novel Rab27A/B effector, should shed light on molecular recognition of its specific cargo by class V myosin.