Analysis of plant gene family heat shock protein 100 (HSP100) and its orthologs in Eukarya reveals sites of divergent evolution and insights into endosymbiotic origins of chloroplasts.

Heat shock protein 100s (HSP100s) is present in bacteria, basal eukaryotes, plants, and fungi, but not in metazoans. While this protein family has been extensively studied in plants in terms of structure and functionality, this research addresses a gap in knowledge about their evolution and origins in eukaryotic groups. Protein sequences were aligned, and phylogenetic relationships were inferred. Amino acid substitution model testing was performed using ProtTest. These results recovered six homologues of plant HSP100s, consistent with previous literature, and also two major lineages of fungal HSP100s. At least one of these two fungal homologues of HSP100 are present in all basal eukaryotic groups studied, indicating their origins at the base of the eukaryotic phylogeny. HSP100 homologs are also present in choanoflagellates (earliest diverging animal lineage), indicating they were later secondarily lost in all other metazoans. Plants have additional 3 copies of HSP100s, obtained from a cyanobacterial ancestor that was the endosymbiotic precursor to the chloroplast. Finally, several groups within the Heterokonts are of particular interest due to serial plastid endosymbiosis, resulting in the presence of some but not all HSP100 proteins in these lineages. This study reveals the complex evolutionary history of this important molecular chaperone protein family.