Integration of Two Ancestral Chaperone Systems into One: The Evolution of Eukaryotic Molecular Chaperones in Light of Eukaryogenesis

Eukaryotic genomes are mosaics of genes acquired from their prokaryotic ancestors, the eubacterial endosymbiont that gave rise to the mitochondrion and its archaebacterial host. Genomic footprints of the prokaryotic merger at the origin of eukaryotes are still discernable in eukaryotic genomes, where gene expression and function correlate with their prokaryotic ancestry. Molecular chaperones are essential in all domains of life as they assist the functional folding of their substrate proteins and protect the cell against the cytotoxic effects of protein misfolding. Eubacteria and archaebacteria code for slightly different chaperones, comprising distinct protein folding pathways. Here we study the evolution of the eukaryotic protein folding pathways following the endosymbiosis event. A phylogenetic analysis of all 64 chaperones encoded in the Saccharomyces cerevisiae genome revealed 25 chaperones of eubacterial ancestry, 11 of archaebacterial ancestry, 10 of ambiguous prokaryotic ancestry, and 18 that may represent eukaryotic innovations. Several chaperone families (e.g., Hsp90 and Prefoldin) trace their ancestry to only one prokaryote group, while others, such as Hsp40 and Hsp70, are of mixed ancestry, with members contributed from both prokaryotic ancestors. Analysis of the yeast chaperone-substrate interaction network revealed no preference for interaction between chaperones and substrates of the same origin. Our results suggest that the archaebacterial and eubacterial protein folding pathways have been reorganized and integrated into the present eukaryotic pathway. The highly integrated chaperone system of yeast is a manifestation of the central role of chaperone-mediated folding in maintaining cellular fitness. Most likely, both archaebacterial and eubacterial chaperone systems were essential at the very early stages of eukaryogenesis, and the retention of both may have offered new opportunities for expanding the scope of chaperone-mediated folding.