Abstract
Protein phosphorylation is one of the widely used posttranslational modifications that alter protein function in vivo. We recently showed phosphorylation of Drosophila Plexin A by cyclic adenosine monophosphate (cAMP)-dependent protein kinase (PKA) and subsequent inhibition of plexin-mediated repulsive guidance. This phosphorylation occurs in the active site of the plexin GTPase-activating protein (GAP) domain, which in turn inhibits endogenous GAP activity toward Ras/Rap family small GTP-binding proteins by recruiting the phospho-serine/threonine-binding protein 14-3-3ε. Here we describe how phosphorylation of Plexin A can be detected and quantitated using an in vitro kinase assay and radioactive [γ-P32] adenosine 5'-triphosphate (ATP).