Rab7 is a critical mediator in vesicular transport of tyrosinase‐related protein 1 in melanocytes

Abstract

How melanosomal proteins such as enzymic proteins (tyrosinase and tyrosinase-related proteins, Tyrps) and structural protein (gp100) are transported from Golgi to melanosomal compartments is not yet fully understood. A number of small GTPases have been found to be associated with melanosomes and we have identified one of them, Rab7, a regulator of vesicular transport, organelle motility, phospholipid signaling and cytosolic degradative machinery, as being involved in the transport of Tyrp1 from Golgi to stage I melanosomes. This study further characterizes the role of Rab7 as a regulator of differential sorting of melanosomal proteins in this process. Murine melanocytes were transiently transfected with a plasmid encoding either wild-type (Rab7WT), constitutively active (Rab7Q67L) or dominant-negative (Rab7N125I and Rab7T22N) Rab7. Through immunocytostaining and confocal laser scanning microscopy, we quantitatively compared the bio-distribution of melanosomal proteins between Rab7WT-expressing cells and mutant Rab7-expressing cells. We also characterized their differential elimination from melanosomal compartments by Rab7 by utilizing a proteasome inhibitor, MG132. Our findings indicate that Rab7 plays an important role in differential sorting of tyrosinase, Tyrp1 and gp100 in early melanogenesis cascade, and that it is more specifically involved with Tyrp1 than tyrosinase and gp100 in the trafficking from Golgi to melanosomes and the specific exit from the degradative process.

Rab4A organizes endosomal domains for sorting cargo to lysosome-related organelles

“…Additionally, several Rabs, such as Rab4, 5, 7, 9, 11 and 22 (Grant and Donaldson, 2009; Pfeffer, 2013; Stenmark, 2009; Zerial and McBride, 2001), and their multiple effectors or tethering proteins, like Rabenosyn-5, Rabaptin-5, Rabip4’, EEA1 and the HOPS complex, have been shown to localize to these domains (de Renzis et al, 2002; Deneka et al, 2003; Fouraux et al, 2004; Ivan et al, 2012; Kalin et al, 2015; Zhu et al, 2009) for the regulation of different cargo transport processes. Furthermore, Rab7 and 9 have been shown to control different transport steps during melanosome biogenesis by functioning on late endosomes (LEs) or melanosomes (Hida et al, 2011; Mahanty et al, 2016). Nonetheless, function of Rab4A in regulating the trafficking of melanocytic cargo during melanosome maturation has not been studied.…”

STX13 regulates cargo delivery from recycling endosomes during melanosome biogenesis

“…Mutations in certain genes that encode multi-subunits protein complexes such as the biogenesis of lysosome-related organelles complexes (BLOC-1, BLOC-2 and BLOC-3) and the adaptor protein (AP)-3 complex, disrupt the cargo transport to melanosomes and other LROs, resulting in Hermansky–Pudlak syndrome (HPS), which is characterized by oculocutaneous albinism, prolonged bleeding and other symptoms ( Di Pietro and Dell'Angelica, 2005 ; Huizing et al, 2008 ; Wei, 2006 ). Moreover, Rabs (Rab7, Rab38 and Rab32), adaptors (AP-1), cytoskeleton regulators (the WASH complex), motor proteins (KIF13A) and Rab effectors (VARP, also known as ANKRD27) also regulate the melanosomal cargo delivery either from recycling endosomal domains or the Golgi in association with HPS protein complexes ( Bultema et al, 2012 ; Delevoye et al, 2009 ; Gerondopoulos et al, 2012 ; Hida et al, 2011 ; Ryder et al, 2013 ; Tamura et al, 2011 ; Wasmeier et al, 2006 ). Previous studies have suggested that the transport of melanosomal proteins to melanosomes occurs in two different routes from recycling endosomes: (1) through BLOC-1-dependent transport of tyrosinase-related protein-1 (TYRP1) and other cargo, and (2) through AP-3-dependent transport of tyrosinase (TYR) to maturing melanosomes containing premelanosomal protein (PMEL) fibrils ( Marks et al, 2013 ; Sitaram and Marks, 2012 ).…”

Rab9A is required for delivery of cargo from recycling endosomes to melanosomes

“…In melanocytes, several Rabs and their effectors, including SNAREs, have been shown to regulate cargo transport to melanosomes. The depletion of Rab7 (Hida et al., 2011 ), VARP (VPS9-ankyrin-repeat protein, an effector protein of Rab38/32) (Tamura et al., 2009 ), or VAMP7 (vesicle-associated membrane protein 7, a binding partner of VARP) (Tamura et al., 2011 ) misroute the BLOC-1-dependent TYRP1 cargo and cause melanocyte hypopigmentation. However, the role of these proteins in TYR (AP-3-dependent) transport has not been addressed.…”