CHAPERONIN-MEDIATED FOLDING OF VERTEBRATE ACTIN-RELATED PROTEIN AND GAMMA-TUBULIN

The folding of actin and tubulin is mediated via interaction with a he teromeric toroidal complex (cytoplasmic chaperonin) that hydrolyzes AT P as part of the reaction whereby native proteins are ultimately relea sed. Vertebrate actin-related protein (actin-RPV) (also termed centrac tin) and gamma-tubulin are two proteins that are distantly related to actin and tubulin, respectively: gamma-tubulin is exclusively located at the centrosome, while actin-RPV is conspicuously abundant at the sa me site. Here we show that actin-RPV and gamma-tubulin are both folded via interaction with the same chaperonin that mediates the folding of beta-actin and alpha- and beta-tubulin. In each case, the unfolded po lypeptide forms a binary complex with cytoplasmic chaperonin and is re leased as a soluble, monomeric protein in the presence of Mg-ATP and t he presence or absence of Mg-GTP. In contrast to alpha- and beta-tubul in, the folding of gamma-tubulin does not require the presence of cofa ctors in addition to chaperonin itself. Monomeric actin-RPV produced i n in vitro folding reactions cocycles efficiently with native brain ac tin, while in vitro folded gamma-tubulin binds to polymerized microtub ules in a manner consistent with interaction with microtubule ends. Bo th monomeric actin-RPV and gamma-tubulin bind to columns of immobilize d nucleotide: monomeric actin-RPV has no marked preference for ATP or GTP, while gamma-tubulin shows some preference for GTP binding. We sho w that actin-RPV and gamma-tubulin compete with one another, and with beta-actin or alpha-tubulin, for binary complex formation with cytopla smic chaperonin.