Individual subunits of the eukaryotic cytosolic chaperonin mediate interactions with binding sites located on subdomains of β-actin

Abstract

The chaperonin containing TCP-1 (CCT) of eukaryotic cytosol is composed of eight different subunit species that are proposed to have independent functions in folding its in vivo substrates, the actins and tubulins. CCT has been loaded with 35S-β-actin by in vitro translation in reticulocyte lysate and then subjected to immunoprecipitation with all eight anti-CCT subunit antibodies in mixed micelle buffers, conditions that disrupt CCT into actin and isolated CCTα, CCTβ, CCTε, or CCTθ subunits are observed, suggesting that polar and electrostatic interactions may mediate actin binding to these four CCT subunits. Additionally, a β-actin peptide array was screened for CCT-binding sequences. Three regions rich in charged and polar amino acid residues, which map to the surface of native β-actin, are implicated in interactions between actin and CCT. Several of these biochemical results are consistent with the recent cryo-electron microscopy three-dimensional structure of apo-CCT-α-actin, in which α-actin is bound by the apical domains of specific CCT subunits. A model is proposed in which actin interacts with several CCT subunits during its CCT-mediated folding cycle.