title: The Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for dendrite formation in melanocytes.
abstract: Vacuolar protein sorting 9 (VPS9)-ankyrin-repeat protein (Varp) has recently been identified as an effector molecule for two small GTPases-Rab32 and Rab38-in the transport of a melanogenic enzyme tyrosinase-related protein 1 (Tyrp1) to melanosomes in melanocytes. Although Varp contains a Rab21-guanine nucleotide exchange factor (GEF) domain (i.e., VPS9 domain), since Rab21-GEF activity is not required for Tyrp1 transport, nothing is known about the physiological significance of the Rab21-GEF activity in melanocytes. Here we show by knockdown-rescue experiments that the Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for forskolin-induced dendrite formation of cultured melanocytes. We found that Varp-deficient cells are unable to extend dendrites in response to forskolin stimulation and that reexpression of wild-type Varp or a Rab32/38-binding-deficient mutant Varp(Q509A/Y550A) in Varp-deficient cells completely restores their ability to form dendrites. By contrast, VPS9 mutants (D310A and Y350A) and a vesicle-associated membrane protein 7 (VAMP7)-binding-deficient mutant were unable to support forskolin-induced dendrite formation in Varp-deficient cells. These findings indicate that the Rab21-GEF activity and Rab32/38 binding activity of Varp are required for different melanocyte functions, that is, Rab21 activation by the VPS9 domain is required for dendrite formation, and the Rab32/38 effector function of the ankyrin repeat 1 domain is required for Tyrp1 transport to melanosomes, although VAMP7-binding ability is required for both functions.
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: Norihiko
authors.initials: N
authors.lastname: Ohbayashi
authors.forename: Ayaka
authors.initials: A
authors.lastname: Yatsu
authors.forename: Kanako
authors.initials: K
authors.lastname: Tamura
authors.forename: Mitsunori
authors.initials: M
authors.lastname: Fukuda
mesh_terms: D000818:Animals; D002460:Cell Line; D003712:Dendrites / Q000378:metabolism / Q000502:physiology*; D055785:Gene Knockdown Techniques; D020662:Guanine Nucleotide Exchange Factors / Q000235:genetics / Q000378:metabolism*; D008544:Melanocytes / Q000378:metabolism / Q000502:physiology* / Q000648:ultrastructure; D051379:Mice; D009154:Mutation; D050683:R-SNARE Proteins / Q000378:metabolism; D020691:rab GTP-Binding Proteins / Q000235:genetics / Q000378:metabolism*
chemical_list: D020662:Guanine Nucleotide Exchange Factors; D050683:R-SNARE Proteins; C496927:Sybl1 protein, mouse; C512038:VPS9-ankyrin-repeat protein, mouse; C478766:Rab32 protein, mouse; C498786:Rab38 protein, mouse; C546392:rab21 protein, mouse; D020691:rab GTP-Binding Proteins