Unique composition of plastid chaperonin-60: α and β polypeptide-encoding genes are highly divergent

Molecular chaperones of the chaperonin family occur in prokaryotes and in plastids and mitochondria. Prokaryotic and mitochondrial chaperonin-60 oligomers (Cpn-60) are composed of a single subunit type (p60 cpn-60 ). In contrast, preparations of purified plastid Cpn-60 contain approximately equal quantities of two polypeptides, p60 cpn-60α and p60 cpn-60β , with slightly different electrophoretic mobilities. We have isolated cDNA clones encoding plastid p60 cpn-60α and p60 cpn-60β polypeptides from Brassica napus and Arabidopsis thaliana. The unexpected degree of sequence divergence observed between p60 cpn-60α and p60 cpn-60β raises questions concerning the structure of the oligomer and the functions of these polypeptides. We have also found an amino acid sequence motif within all p60 cpn-60 sequences which resembles the p10 cpn-10 sequences.