Structures and co-regulated expression of the genes encoding mouse cytosolic chaperonin CCT subunits

The chaperonin-containing TCP-1 (CCT) is a hetero-oligomeric molecular chaperone that mediates protein folding in the cytosol of eukaryotes. Eight (or nine in testis) subunit species are assembled in the CCT hexadecamer complex. We have cloned seven CCT subunit genes, Cctb, Cctd, Ccte, Cctz-1, Cctz-2 (testis specific), Ccth and Cctq, from mouse genomic DNA libraries, in addition to the Ccta and Cctg genes reported previously, and the entire nucleotide sequences of these DNA clones were determined. These genes are approximately 15-20 kb in length except for Cctz-2 which is longer than 35 kb, and all the Cct genes consist of 11-16 exons. Primer extension analyses of testis RNA indicate one to several potential transcription start sites 50-150 bp upstream from the translation start codon of each Cct gene. There are several possible Sp1-binding sequences, but no obvious TATA box was observed around the potential start sites. From 5'-flanking regions to the first introns, the Cct genes are rich in CpG dinucleotides. In reporter gene assays using these regions, five of eight Cct genes showed strong transcriptional activity comparable with the combination of SV40 promoter and enhancer in HeLa cells. We also show, by Western and Northern blot analyses, that CCT expression levels vary widely among different tissues but the expression patterns are very similar among the eight subunit species. It is likely that expression levels of the eight different subunits are tightly co-regulated to maintain a constant ratio of these subunits which constitute the CCT hexadecamer complex with a fixed subunit arrangement.