PHYLOGENETIC ANALYSIS OF THE 90 KD HEAT-SHOCK FAMILY OF PROTEIN SEQUENCES AND AN EXAMINATION OF THE RELATIONSHIP AMONG ANIMALS, PLANTS, ANDFUNGI SPECIES

The heat shock protein (Hsp) sequences;because of their ubiquity and h igh degree of conservation, provide useful models for phylogenetic ana lysis. In this paper I have carried out a global alignment of all avai lable sequences (a total of 31) for the 90-kD heat shock protein (Hsp9 0) family. The minimum amino acid identity that is seen between presen tly known Hsp90 homologs is about 40% over the entire length, indicati ng that it is a highly conserved protein. Based on the alignment, a nu mber of signature sequences that either are distinctive of the Hsp90 f amily or that distinguish between the cytosolic and the endoplasmic re ticular forms of Hsp90 have been identified. Detailed phylogenetic ana lyses based on Hsp90 sequences reported here strongly indicate that th e cytosolic and the endoplasmic reticulum (ER) resident forms of Hsp90 constitute paralogous gene families which arose by a gene duplication event that took place very early in the evolution of eukaryotic cells . A minimum of two additional gene duplication events, which took plac e at a later time, are required to explain the presence of two differe nt forms of Hsp90 that are found in fungi and vertebrate species. In a consensus neighbor-joining bootstrap tree based on Hsp90 sequences, p lants and animals species grouped together 989 times of 1,000 (a highl y significant score), indicating a closer relationship between them as compared to fungi. A closer affiliation of plant and animal species w as also observed in the maximum-parsimony tree, although the relations hip was not significantly supported by this method. A survey of the re cent literature an this subject indicates that depending on the protei n sequence and the methods of phylogenetic analysis, the animal specie s are indicated as closer relatives to either plants or fungi with sig nificant statistical support for both topologies. Thus the relationshi p among the animal, plant, and fungi kingdoms remains an unresolved is sue at the present time.