Chaperonin genes on the rise: new divergent classes and intense duplication in human and other vertebrate genomes

Chaperonin proteins are well known for the critical role they play in protein folding and in disease. However, the recent identification of three diverged chaperonin paralogs associated with the human Bardet-Biedl and McKusick-Kaufman Syndromes (BBS and MKKS, respectively) indicates that the eukaryotic chaperonin-gene family is larger and more differentiated than previously thought. The availability of complete genome sequences makes possible a definitive characterization of the complete set of chaperonin sequences in human and other species.

We identified fifty-four chaperonin-like sequences in the human genome and similar numbers in the genomes of the model organisms mouse and rat. In mammal genomes we identified, besides the well-known CCT chaperonin genes and the three genes associated with the MKKS and BBS pathological conditions, a newly-defined class of chaperonin genes named CCT8L, represented in human by the two sequences CCT8L1 and CCT8L2. Comparative analyses from several vertebrate genomes established the monophyletic origin of chaperonin-like MKKS and BBS genes from the CCT8 lineage. The CCT8L gene originated from a later duplication also in the CCT8 lineage at the onset of mammal evolution and duplicated in primate genomes. The functionality of CCT8L genes in different species was confirmed by evolutionary analyses and in human by expression data. Detailed sequence analysis and structural predictions of MKKS, BBS and CCT8L proteins strongly suggested that they conserve a typical chaperonin-like core structure but that they are unlikely to form a CCT-like oligomeric complex. The characterization of many newly-discovered chaperonin pseudogenes uncovered the intense duplication activity of eukaryotic chaperonin genes.

In vertebrates, chaperonin genes, driven by intense duplication processes, have diversified into multiple classes and functionalities that extend beyond their well-known protein-folding role as part of the typical oligomeric chaperonin complex, emphasizing previous observations on the involvement of individual CCT monomers in microtubule elongation. The functional characterization of newly identified chaperonin genes will be a challenge for future experimental analyses.