Prefoldin recognition motifs in the nonhomologous proteins of the actin and tubulin families.

ABSTRACT

Nascent actin and tubulin molecules undergo a series of complex interactions with chaperones and are thereby guided to their native conformation. These cytoskeletal proteins have the initial part of the pathway in common both interact with prefoldin and with the cytosolic chaperonin containing tailless complex polypeptide 1. Little is understood with regard to how these chaperones and, in particular, prefoldin recognize the non-native forms of these target proteins. Using mutagenesis, we provide evidence that beta-actin and alpha-tubulin each have two prefoldin interaction sites. The most amino-terminally located site of both proteins shows striking sequence similarity, although these proteins are nonhomologous. Very similar motifs are present in beta- and gamma-tubulin and in the newly identified prefoldin target protein actin-related protein 1. Actin-related proteins 2 and 3 have related motifs, but these have altered charge properties. The latter two proteins do not bind prefoldin, although we identify them here as target proteins for the cytosolic chaperonin. Actin fragments containing the two prefoldin interaction regions compete efficiently with actin for prefoldin binding. In addition, they also compete with tubulins, suggesting that these target proteins contact similar prefoldin subunits.