GAPDH binds Akt to facilitate cargo transport in the early secretory pathway

Glyceraldehyde-3-phosphate dehydrogenase (GAPDH) undergoes numerous post-translational modifications, which impart new function and influence intracellular location. For example, atypical PKC ι/λ phosphorylates GAPDH that locates to vesicular tubular clusters and is required for retrograde membrane trafficking in the early secretory pathway. GAPDH is also required in the endocytic pathway; substitution of Pro234 to Ser (Pro234Ser) rendered CHO cells defective in endocytosis. To determine if GAPDH (Pro234Ser) could inhibit endoplasmic reticulum to Golgi trafficking, we introduced the recombinant mutant enzyme into several biochemical and morphological transport assays. The mutant protein efficiently blocked vesicular stomatitis virus-G protein transport. Because GAPDH binds to microtubules (MTs), we evaluated MT binding and MT intracellular distribution in the presence of the mutant. Although these properties were not changed relative to wild-type, GAPDH (Pro234Ser) altered Golgi complex morphology. We determined that the GAPDH point mutation disrupted association between the enzyme and the serine/threonine kinase Akt. Interestingly Rab1, which functions in anterograde-directed trafficking, stimulates GAPDH-Akt association with membranes in a quantitative binding assay. In contrast, Rab2 does not stimulate GAPDH-Akt membrane binding but instead recruits GAPDH-aPKC. We propose a mechanism whereby the association of GAPDH with Akt or with aPKC serves as a switch to discriminate between anterograde directed cargo and recycling cargo retrieved back to the ER, respectively.