The Rab21-GEF activity of Varp, but not its Rab32/38 effector function, is required for dendrite formation in melanocytes.

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.