IDENTIFICATION OF A NOVEL CORTACTIN SH3 DOMAIN-BINDING PROTEIN AND ITS LOCALIZATION TO GROWTH CONES OF CULTURED NEURONS

Cortactin is an actin-binding protein that contains several potential signaling motifs including a Src homolog 3 (SH3) domain at the distal C terminus. Translocation of cortactin to specific cortical actin stru ctures and hyperphosphorylation of cortactin on tyrosine have been ass ociated with the cortical cytoskeleton reorganization induced by a var iety of cellular stimuli. The function of cortactin in these processes is largely unknown in part due to the lack of information about cellu lar binding partners for cortactin. Here we report the identification of a novel cortactin-binding protein of approximately 180 kDa by yeast two-hybrid interaction screening. The interaction of cortactin with t his 180-kDa protein was confirmed by both in vitro and in vivo methods , and the SH3 domain of cortactin was found to direct this interaction . Since this protein represents the first reported natural ligand for the cortactin SH3 domain, we designated it CortBP1 for cortactin-bindi ng protein 1. CortBP1 contains two recognizable sequence moths within its C-terminal region, including a consensus sequence for cortactin SH 3 domain-binding peptides and a sterile alpha motif. Northern and West ern blot analysis indicated that CortBP1 is expressed predominately in brain tissue. Immunofluorescence studies revealed colocalization of C ortBP1 with cortactin and cortical actin filaments in lamellipodia and membrane ruffles in fibroblasts expressing CortBP1. Colocalization of endogenous CortBP1 and cortactin was also observed in growth cones of developing hippocampal neurons, implicating CortBP1 and cortactin in cytoskeleton reorganization during neurite outgrowth.