Molecular chaperones assist in the folding and assembly of proteins in cells. Although chaperonins have been shown in prokaryotes, mitochondria, and chloroplasts long-ago, a cytoplasmic heteromeric chaperonin complex was isolated only recently and found to contain at least five to six polypeptides, one of which was identified as the product of the T complex polypeptide-1 (TCP-1) gene. We have isolated and cloned a novel gene called A45 from a cDNA library constructed from poly (A)+ RNA of a multipotent hematopoietic progenitor clone. The A45 cDNA encodes a predicted polypeptide of M(r) 58,118 that exhibits 32% overall amino acid sequence identity to TCP-1 and contains the putative adenosine triphosphate-binding domain and two characteristic consensus regions that are conserved in all chaperonins. The A45 gene is expressed in hematopoietic precursors cells at a much higher level than in nonhematopoietic cells and tissues. We conclude that A45 represents a new member of the mammalian chaperonins that is involved in the folding and assembly of polypeptides.

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