title: Varp is a novel Rab32/38-binding protein that regulates Tyrp1 trafficking in melanocytes.
abstract: Two small GTPase Rabs, Rab32 and Rab38, have recently been proposed to regulate trafficking of melanogenic enzymes to melanosomes in mammalian epidermal melanocytes; however, the exact molecular mechanism of Rab32/38-mediated transport of melanogenic enzymes has never been clarified, because no Rab32/38-specific effector has ever been identified. In this study, we screened for a Rab32/38-specific effector by a yeast two-hybrid assay using a guanosine triphosphate (GTP)-locked Rab32/38 as bait and found that VPS9-ankyrin-repeat protein (Varp)/Ankrd27, characterized previously as a guanine nucleotide exchange factor (GEF) for Rab21, functions as a specific Rab32/38-binding protein in mouse melanocyte cell line melan-a. Deletion analysis showed that the first ankyrin-repeat (ANKR1) domain functions as a GTP-dependent Rab32/38-binding domain, but that the N-terminal VPS9 domain (i.e., Rab21-GEF domain) does not. Small interfering RNA-mediated knockdown of endogenous Varp in melan-a cells caused a dramatic reduction in Tyrp1 (tyrosinase-related protein 1) signals from melanosomes but did not cause any reduction in Pmel17 signals. Furthermore, expression of the ANKR1 domain in melan-a cells also caused a dramatic reduction of Tyrp1 signals, whereas the VPS9 domain had no effect. Based on these findings, we propose that Varp functions as the Rab32/38 effector that controls trafficking of Tyrp1 in melanocytes.
authors.affiliation: Laboratory of Membrane Trafficking Mechanisms, Department of Developmental Biology and Neurosciences, Graduate School of Life Sciences, Tohoku University, Aobayama, Aoba-ku, Sendai, Miyagi 980-8578, Japan.
authors.forename: Kanako
authors.initials: K
authors.lastname: Tamura
authors.forename: Norihiko
authors.initials: N
authors.lastname: Ohbayashi
authors.forename: Yuto
authors.initials: Y
authors.lastname: Maruta
authors.forename: Eiko
authors.initials: E
authors.lastname: Kanno
authors.forename: Takashi
authors.initials: T
authors.lastname: Itoh
authors.forename: Mitsunori
authors.initials: M
authors.lastname: Fukuda
mesh_terms: D000818:Animals; D001665:Binding Sites; D002460:Cell Line; D020662:Guanine Nucleotide Exchange Factors / Q000737:chemistry / Q000172:deficiency / Q000378:metabolism*; D008544:Melanocytes / Q000166:cytology / Q000201:enzymology*; D008562:Membrane Glycoproteins / Q000378:metabolism*; D051379:Mice; D008969:Molecular Sequence Data; D010088:Oxidoreductases / Q000378:metabolism*; D011485:Protein Binding; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D034741:RNA, Small Interfering / Q000378:metabolism; D020798:Two-Hybrid System Techniques; D020691:rab GTP-Binding Proteins / Q000378:metabolism*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D008562:Membrane Glycoproteins; D034741:RNA, Small Interfering; C512038:VPS9-ankyrin-repeat protein, mouse; D010088:Oxidoreductases; C079621:Tyrp1 protein, mouse; C478766:Rab32 protein, mouse; C498786:Rab38 protein, mouse; D020691:rab GTP-Binding Proteins