Association of mouse actin-binding protein 1 (mAbp1/SH3P7), an Src kinase target, with dynamic regions of the cortical actin cytoskeleton in responseto Rac1 activation

Yeast Abp1p is a cortical actin cytoskeleton protein implicated in cytoskel etal regulation, endocytosis, and cAMP-signaling. We have identified a gene encoding a mouse homologue of Abp1p, and it is identical to SH3P7, a prote in shown recently to be a target of Src tyrosine kinases. Yeast and mouse A bp1p display the same domain structure including an N-terminal actin-depoly merizing factor homology domain and a C-terminal Src homology 3 domain. Usi ng two independent actin-binding domains, mAbp1 binds to actin filaments wi th a 1:5 saturation stoichiometry. In stationary cells, mAbp1 colocalizes w ith cortical F-actin in fibroblast protrusions that represent sites of cell ular growth. mAbp1 appears at the actin-rich leading edge of migrating cell s. Growth factors cause mAbp1 to rapidly accumulate in lamellipodia. This r esponse can be mimicked by expression of dominant-positive Rad. mAbp1 recru itment appears to be dependent on de novo actin polymerization and occurs s pecifically at sites enriched for the Arp2/3 complex. mAbp1 is a newly iden tified cytoskeletal protein in mice and may serve as a signal-responsive li nk between the dynamic cortical actin cytoskeleton and regions of membrane dynamics.