Fyn promotes phosphorylation of collapsin response mediator protein 1 at tyrosine 504, a novel, isoform-specific regulatory site.

In vertebrates the collapsin response mediator proteins (CRMPs) are encoded by five highly related genes. CRMPs are cytosolic phosphoproteins abundantly expressed in developing and mature mammalian brains. CRMPs are best understood as effectors of Semaphorin 3A signaling regulating growth cone collapse in migratory neurons. Phosphorylation in the carboxyl-terminal regulatory domain of CRMPs by several serine/threonine kinases has been described. These phoshorylation events appear to function, at least in part, to disrupt the interaction of CRMPs with tubulin heterodimers. In a large-scale phosphoproteomic analysis of murine brain, we recently identified a number of in vivo tyrosine phosphorylation sites on CRMP isoforms. Using biochemical approaches and quantitative mass spectrometry we demonstrate that one of these sites, CRMP1 tyrosine 504 (Y504), is a primary target of the Src family of tyrosine kinases (SFKs), specifically Fyn. Y504 is adjacent to CDK5 and GSK-3beta sites that regulate the interaction of CRMPs with tubulin. Although Y504 is highly conserved among vertebrate CRMP1 orthologs, a residue corresponding to Y504 is absent in CRMP isoforms 2-5. This suggests an isoform-specific regulatory role for CRMP1 Y504 phosphorylation and may help explain the observation that CRMP1-deficient mice exhibit neuronal migration defects not compensated for by CRMPs 2-5.