Rab40C is a novel Varp-binding protein that promotes proteasomal degradation of Varp in melanocytes

Abstract

Varp (VPS9-ankyrin repeat protein) was originally identified as an activator of small GTPase Rab21 through its VPS9 domain, but it has subsequently been shown to function as a Rab32/38 effector through its first ANKR1 domain. Although these functions of Varp are important for melanogenesis, Varp contains a second ANKR2 domain, whose function remained completely unknown. Here we identified Rab40C, an atypical Rab containing a SOCS box that recruits a ubiquitin ligase complex, as a novel ANKR2-binding protein and investigated its involvement in melanogenic enzyme trafficking in melanocytes. The results showed that overexpression of Rab40C in melanocytes caused a dramatic reduction in melanogenic enzyme Tyrp1 signals by promoting proteasomal degradation of Varp in a SOCS-box-dependent manner and that knockdown of Rab40C in melanocytes caused an increase in the amount of Varp. Intriguingly, Rab40C knockdown also caused a dramatic reduction in Tyrp1 signals, the same as Varp overexpression did. These findings indicated that Rab40C is a previously unexpected regulator of Tyrp1 trafficking in melanocytes through controlling the proteasomal degradation of Varp.

Rab35 GTPase recruits NDP52 to autophagy targets

“…However, galectin 8 binding is not required for NDP52 recruitment to GAS, in contrast to the cellular response against Salmonella. We note that a similar observation was recently reported for Rab40C, overexpression and knockdown of which both repressed production of melanin (Yatsu et al, 2015). Importantly, these experiments also demonstrated that Rab35 and its inactivation by TBC1D10A regulate autophagosome biogenesis by recruiting NDP52 to invading bacteria.…”

Multiple Roles of VARP in Endosomal Trafficking: Rabs, Retromer Components and R‐SNARE VAMP7 Meet on VARP

“…A) Schematic representation of VARP and its ligand binding sites. The VPS9 domain, ANKR1 domain, ANKR2 domain, VID domain and two Cys‐rich regions are responsible for GDP‐Rab21 binding , GTP‐Rab32/38 binding , GTP‐Rab40C binding , VAMP7 binding and VPS29 binding , respectively. The precise KIF5A, GOLGA4 and VPS35 binding sites have not yet been determined .…”

“…Consequently, beads coupled with the recombinant ANKR1 domain are a useful tool for trapping active Rab32/38 in cell lysates , and when the ANKR1 domain is expressed in melanocytes, it functions as a ‘GTP‐Rab32/38 trapper’ that specifically inhibits the function of Rab32/38 . Although the ANKR1 domain and ANKR2 domain exhibit more than 60% amino acid similarity, the ANKR2 domain of VARP does not recognize Rab32/38, and instead specifically binds to the GTP‐bound active form of Rab40 subfamily members . However, the ANKR2 domain is unlikely to function as a Rab40 effector domain, and Rab40 promotes proteasomal degradation of VARP by recruiting an E3 ubiquitin ligase complex via its SOCS domain (; see next section for details).…”

Multiple Types of Guanine Nucleotide Exchange Factors (GEFs) for Rab Small GTPases

“…Rab22B/31 (Kajiho et al, 2011) 4 ALS2 ANKRD27 (84079) Varp Rab21 (Zhang et al, 2006) 2,3 (Tamura et al, 2011) 3 Rab32 (Tamura et al, 2009) Rab38 (Wang et al, 2008) VAMP7 (Burgo et al, 2009) VPS29 (Hesketh et al, 2014) VPS35 (McGough et al, 2014) Rab40 (Yatsu et al, 2015) RACK1 (Marubashi et al, 2016b) VPS9D1 ( Rab8 (Hattula et al, 2002) 1,2,3…”

“…Rab21-activation by Varp and its interaction with VAMP7 cooperate to regulate neurite outgrowth of both PC12 cells and mouse hippocampal neurons and dendrite outgrowth of melanocytes (Burgo et al, 2009(Burgo et al, , 2012Ohbayashi et al, 2012). Varp is also involved in melanogenic enzyme transport to melanosomes, which are known as lysosome-related organelles, through interaction with Rab32 and Rab38 via the ANKR1 domain (Tamura et al, 2009(Tamura et al, , 2011 and with Rab40C via the ANKR2 domain (Yatsu et al, 2015). Moreover, Varp interacts with two subunits of the retromer complex (VPS29 and VPS35) and is involved in the endosome-to-plasma-membrane recycling of certain transmembrane proteins (Hesketh et al, 2014;McGough et al, 2014).…”