Src-dependent aProtein Kinase C / (aPKC / ) Tyrosine Phosphorylation Is Required for aPKC / Association with Rab2 and Glyceraldehyde-3-phosphate Dehydrogenase on Pre-Golgi Intermediates*

The small GTPase Rab2 is required for membrane transport between the endoplasmic reticulum (ER) and the Golgi complex. Rab2 associateswith pre-Golgi intermediates (also termed vesicular tubular clusters; VTCs) that sort cargo to the anterograde pathway from recycling proteins retrieved to the ER. Our previous studies have shown that Rab2 stimulates atypical protein kinase C / (aPKC / ) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) recruitment to VTCs. Both aPKC / and GAPDH bind directly to Rab2 and aPKC / and GAPDH interact. Based on the reports demonstrating aPKC -Src interaction andSrc activity in the retrograde pathway (Golgi-ER), studies were initiated to learn whether Rab2 also promoted Src recruitment to VTCs. Using a quantitative membrane binding assay, we found that Rab2-stimulated Src membrane association in a dose-dependent manner. The recruited Src binds to aPKC / and GAPDH on the membrane; however, Src does not interact with Rab2. The membrane-associated Src tyrosine phosphorylates aPKC / on the VTC. To determine the consequence of aPKC / tyrosine phosphorylation, the membrane binding assay was supplemented with the Src-specific tyrosine kinase inhibitor 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo(3,4-d)pyrimidine (PP2). Although Rab2, Src, and GAPDH recruitment was not affected, the Rab2-PP2-treatedmembranes contained a negligible amount of aPKC / . Since Rab2 requires aPKC / for the downstream recruitment of -coat protein ( -COP) to VTCs, the Rab2-PP2-treated membranes were evaluated for thepresence of -COP. Like aPKC / , themembranes contained a negligible amount of -COP that was reflected by the drastic reduction in Rab2-dependent vesicle formation. These data suggest that Src-mediated tyrosine phosphorylation of aPKC / facilitates aPKC / association with Rab2-Src-GAPDH on VTCs, which is ultimately necessary for the downstream recruitment of -COP and release of Rab2-mediated retrograde-directed vesicles.