THE 8TH CCT GENE, CCTQ, ENCODING THE THETA-SUBUNIT OF THE CYTOSOLIC CHAPERONIN CONTAINING TCP-1

The chaperonin containing t-complex polypeptide 1 (TCP-1), as one of its subunits, CCT, is a cytosolic heterooligomeric molecular chaperone assisting in the folding of proteins in eukaryotic cytosol. We have isolated a Tcp-1-related 119-bp cDNA fragment from a human cDNA library by polymerase chain reaction, and cloned full-length mouse cDNAs orthologous to the human cDNA by hybridization. The nucleotide (nt) sequence of the longest mouse clone (1844 bp) shows an open reading frame (ORF) encoding a TCP-1-related polypeptide of 548 amino acids (aa) (59 562 Da). This gene is different from Tcp-1 and the six Tcp-1-related genes reported previously, Tcp-1 (Ccta), Cctb, Cctg, Cctd, Ccte, Cctz and Ccth, which encode subunits of CCT. The product of the novel gene was analysed using an antibody raised against the C terminus of the polypeptide deduced from the nt sequence. We found that this gene encodes a subunit of CCT (polypeptide S1; 62 kDa and pi 6.25 by two-dimensional gel analysis). We have named it Cctq, encoding the a subunit of CCT (CCTB). The aa sequence of CCTB shows 23-29% identity to the other CCT subunits, alpha, beta, gamma, delta, epsilon, xi and eta, and 29% identity to the archaebacterial chaperonin TF55. CCTB also contains the motifs common to all the other subunits of CCT which are postulated to be involved in ATPase activity.