MOLECULAR EVOLUTION OF THE HSP70 MULTIGENE FAMILY

Eukaryotic genomes encode multiple 70-kDa heat-shock proteins (HSP70s) . The Saccharomyces cerevisiae HSP70 family is comprised of eight memb ers. Here we present the nucleotide sequence of the SSA3 and SSB2 gene s, completing the nucleotide sequence data for the yeast HSP70 family. We have analyzed these yeast sequences as well as 29 HSP70s from 24 a dditional eukaryotic and prokaryotic species. Comparison of the sequen ces demonstrates the extreme conservation of HSP70s; proteins from the most distantly related species share at least 45% identity and more t han one-sixth of the amino acids are identical in the aligned region ( 567 amino acids) among all proteins analyzed. Phylogenetic trees const ructed by two independent methods indicate that ancient molecular and cellular events have given rise to at least four monophyletic groups o f eukaryotic HSP70 proteins. Each group of evolutionarily similar HSP7 0s shares a common intracellular localization and is presumed to be co mprised of functional homologues; these include heat-shock proteins of the cytoplasm, endoplasmic reticulum, mitochondria, and chloroplasts. HSP70s localized in mitochondria and plastids are most similar to the DnaK HSP70 homologues in purple bacteria and cyanobacteria, respectiv ely, which is consistent with the proposed prokaryotic origin of these organelles. The analyses indicate that the major eukaryotic HSP70 gro ups arose prior to the divergence of the earliest eukaryotes, roughly 2 billion years ago. In some cases, as exemplified by the SSA genes en coding the cytoplasmic HSP70s of S. cerevisiae, more recent duplicatio n events have given rise to subfamilies within the major groups. The S . cerevisiae SSB proteins comprise a unique subfamily not identified i n other species to date. This subfamily appears to have resulted from an ancient gene duplication that occurred at approximately the same ti me as the origin of the major eukaryotic HSP70 groups.