Phylogenetic relationships of organellar Hsp90 homologs reveal fundamental differences to organellar Hsp70 and Hsp60 evolution

Abstract

In agreement with endosymbiont theory for the origin of organelles, mitochondria and chloroplasts (plastids) are universally accepted to have monophyletically arisen from within α-proteobacteria and cyanobacteria, respectively. Convincing particular evidence in support of this theory emerged from phylogenetic analysis of highly conserved, ubiquitous heat shock proteins (Hsps) chaperonin 60 and Hsp70. These apparently indispensable general chaperones have proven to be highly useful molecular tracers of organellar origin. Phylogenetic relationships of Hsp90 – a less conserved and less widely distributed general chaperone – are reported here that are strikingly incongruent with canonical patterns of endosymbiotic ancestry. It appears that Hsp90 of chloroplasts derives from the endoplasmic reticulum-specific isoform while mitochondrial Hsp90 homologs affiliate with a eubacterial lineage other than α subdivision of proteobacteria. These data suggest that endosymbiont htpG genes, encoding Hsp90, have been either functionally displaced by pre-existing nuclear genes or completely lost during establishment of organelles and subsequently added to initial organellar complement.