CLONING OF GIARDIA-LAMBLIA HEAT-SHOCK PROTEIN HSP70 HOMOLOGS - IMPLICATIONS REGARDING ORIGIN OF EUKARYOTIC CELLS AND OF ENDOPLASMIC-RETICULUM

The genes for two different 70-kDa heat shock protein (HSP70) homologs have been cloned and sequenced from the protozoan Giardia lamblia. On the basis of their sequence features, one of these genes corresponds to the cytoplasmic form of HSP70. The second gene, on the basis of its characteristic N-terminal hydrophobic signal sequence and C-terminal endoplasmic reticulum (ER) retention sequence (Lys-Asp-Glu-Leu), is th e equivalent of ER-resident GRP78 or the Bip family of proteins. Phylo genetic trees based on HSP70 sequences show that G. lamblia homologs s how the deepest divergence among eukaryotic species. The identificatio n of a GRP78 or Bip homolog in G. lamblia strongly suggests the existe nce of ER in this ancient eukaryote. Detailed phylogenetic analyses of HSP70 sequences by boot-strap neighbor-joining and maximum-parsimony methods show that the cytoplasmic and ER homologs form distinct subfam ilies that evolved from a common eukaryotic ancestor by gene duplicati on that occurred very early in the evolution of eukaryotic cells. It i s postulated that because of the essential ''molecular chaperone'' fun ction of these proteins in translocation of other proteins across memb ranes, duplication of their genes accompanied the evolution of ER or n ucleus in the eukaryotic cell ancestor. The presence in all eukaryotic cytoplasmic HSP70 homologs (including the cognate, heat-induced, and ER forms) of a number of autapomorphic sequence signatures that are no t present in any prokaryotic or organellar homologs provides strong ev idence regarding the monophyletic nature of eukaryotic lineage. Furthe r, all eukaryotic HSP70 homologs share in common with the Gram-negativ e group of eubacteria a number of sequence features that are not prese nt in any archaebacterium or Gram-positive bacterium, indicating their evolution from this group of organisms. Some implications of these fi ndings regarding the evolution of eukaryotic cells and ER are discusse d.