A CONSERVED PROLINE-RICH REGION OF THE SACCHAROMYCES-CEREVISIAE CYCLASE-ASSOCIATED PROTEIN BINDS SH3 DOMAINS AND MODULATES CYTOSKELETAL LOCALIZATION

Saccharomyces cerevisiae cyclase-associated protein (CAP or Srv2p) is multifunctional, The N-terminal third of CAP binds to adenylyl cyclase and has been implicated in adenylyl cyclase activation in vivo. The w idely conserved C-terminal domain of CAP binds to monomeric actin and serves an important cytoskeletal regula tory function in vivo. In addi tion, all CAP homologs contain a centrally located proline-rich region which has no previously identified function, Recently, SH3 (Src homol ogy 3) domains were shown to bind to proline-rich regions of proteins, Here we report that the proline-rich region of CAP is recognized by t he SH3 domains of several proteins, including the yeast actin-associat ed protein Abp1p. Immunolocalization experiments demonstrate that CAP colocalizes with cortical actin-containing structures in vivo and that a region of CAP containing the SH3 domain binding site is required fo r this localization. We also demonstrate that the SH3 domain of yeast Abp1p and that of the yeast RAS protein guanine nucleotide exchange fa ctor Cdc25p complex with adenylyl cyclase in vitro. Interestingly, the binding of the Cdc25p SH3 domain is not mediated by CAP and therefore may involve direct binding to adenylyl cyclase or to an unidentified protein which complexes with adenylyl cyclase, We also found that CAP homologs from Schizosaccharomyces pombe and humans bind SH3 domains. T he human protein binds most strongly to the SH3 domain from the abl pr oto-oncogene. These observations identify CAP as an SH3 domain-binding protein and suggest that CAP mediates interactions between SH3 domain proteins and monomeric actin.