2 SEPARATE FUNCTIONS ARE ENCODED BY THE CARBOXYL-TERMINAL DOMAINS OF THE YEAST CYCLASE-ASSOCIATED PROTEIN AND ITS MAMMALIAN HOMOLOGS - DIMERIZATION AND ACTIN-BINDING

The yeast adenylyl cyclase-associated protein, CAP, was identified as a component of the RAS-activated cyclase complex. CAP consists of two functional domains separated by a proline-rich region, One domain, whi ch localizes to the amino terminus, mediates RAS signaling through ade nylyl cyclase, while a domain at the carboxyl terminus is involved in the regulation of cell growth and morphogenesis. Recently, the carboxy l terminus of yeast CAP was shown to sequester actin, but whether this function has been conserved, and is the sole function of this domain, is unclear, Here, we demonstrate that the carboxyl-terminal domains o f CAP and CAP homologs have two separate functions, We show that carbo xyl-terminals of both yeast CAP and a mammalian CAP homolog, MCH1, bin d to actin, We also show that this domain contains a signal for dimeri zation, allowing both CAP and MCH1 to form homodimers and heterodimers . The properties of actin binding and dimerization are mediated by sep arate regions on the carboxyl terminus; the last 27 amino acids of CAP being critical for actin binding, Finally, we present evidence that l inks a segment of the proline rich region of CAP to its localization i n yeast, Together, these results suggest that all three domains of CAP proteins are functional.