title: Eukaryotic type II chaperonin CCT interacts with actin through specific subunits.
abstract: Chaperonins assist the folding of other proteins. Type II chaperonins, such as chaperonin containing TCP-1(CCT), are found in archaea and in the eukaryotic cytosol. They are hexadecameric or nonadecameric oligomers composed of one to eight different polypeptides. Whereas type I chaperonins like GroEL are promiscuous, assisting in the folding of many other proteins, only a small number of proteins, mainly actin and tubulin, have been described as natural substrates of CCT. This specificity may be related to the divergence of the eight CCT subunits. Here we have obtained a three-dimensional reconstruction of the complex between CCT and alpha-actin by cryo-electron microscopy and image processing. This shows that alpha-actin interacts with the apical domains of either of two CCT subunits. Immunolabelling of CCT-substrate complexes with antibodies against two specific CCT subunits showed that actin binds to CCT using two specific and distinct interactions: the small domain of actin binds to CCTdelta and the large domain to CCTbeta or CCTepsilon (both in position 1,4 with respect to delta). These results indicate that the binding of actin to CCT is both subunit-specific and geometry-dependent. Thus, the substrate recognition mechanism of eukaryotic CCT may differ from that of prokaryotic GroEL.
authors.affiliation: Centro Nacional de Biotecnologia, C.S.I.C., Campus Universidad Autónoma de Madrid, Spain.
authors.forename: O
authors.initials: O
authors.lastname: Llorca
authors.forename: E A
authors.initials: EA
authors.lastname: McCormack
authors.forename: G
authors.initials: G
authors.lastname: Hynes
authors.forename: J
authors.initials: J
authors.lastname: Grantham
authors.forename: J
authors.initials: J
authors.lastname: Cordell
authors.forename: J L
authors.initials: JL
authors.lastname: Carrascosa
authors.forename: K R
authors.initials: KR
authors.lastname: Willison
authors.forename: J J
authors.initials: JJ
authors.lastname: Fernandez
authors.forename: J M
authors.initials: JM
authors.lastname: Valpuesta
mesh_terms: D000199:Actins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D000818:Animals; D001665:Binding Sites; D056404:Chaperonin Containing TCP-1; D018833:Chaperonins / Q000378:metabolism*; D003001:Cloning, Molecular; D004926:Escherichia coli; D006801:Humans; D008958:Models, Molecular; D011487:Protein Conformation; D051381:Rats; D011993:Recombinant Fusion Proteins / Q000378:metabolism
chemical_list: D000199:Actins; C533298:CCT2 protein, human; C533300:CCT4 protein, human; C533301:CCT5 protein, human; D011993:Recombinant Fusion Proteins; D056404:Chaperonin Containing TCP-1; D018833:Chaperonins