Functional significance of Hsp70 post-translational modification in prion propagation and cellular function

The term prion (proteinaceous infectious particles) was first coined by Stanley Prusiner while naming the causative agent responsible for a group of invariably fatal neurodegenerative diseases collectively termed transmissible spongiform encephalopathies (TSE). A breakthrough in prion research came with the studies which revealed that yeast species Saccharomyces cerevisiae contains proteins that have the ability to form prions. Sup35 is a S. cerevisiae protein involved in termination of translation. In a prion state referred to as [PSI+], a significant portion of the Sup35 protein in the cell coalesces into non-functional, self-propagating, amyloid-like polymers. Thus, yeast strains that are [PSI+] show increased levels of nonsense suppression. Once present, [PSI+] propagates by recruitment of the soluble form of Sup35 into the aggregate in a manner analogous to that of mammalian prions. A search for genetic factors affecting propagation and maintenance of [PSI+] has identified an essential role for molecular chaperones, namely Hsp70 and Hsp104. The Hsp70 chaperone family and its associated co-chaperones are highly conserved from yeast to mammals. A major function of Hsp70 is to prevent the aggregation of denatured proteins by binding to exposed hydrophobic regions and preventing the accumulation of amorphous aggregates. In the model eukaryotic S. cerevisiae, to efficiently carry out such functions Hsp70 works in concert with a number of co-chaperones to regulate ATPase hydrolysis cycle of Hsp70, which in-turn dictates the peptide-binding status of Hsp70. While much data has accrued in relation to the ATPase and substrate binding cycles of Hsp70 there is a distinct lack of information regarding the regulation of this important chaperone at the post-translational level. Recent global proteomic studies have demonstrated that in vivo Hsp70 is phosphorylated. Using a simplified yeast system this study systematically assessed a variety of non-phosphorylateable and phosphomimetic Hsp70 mutants for phenotypic alterations in Hsp70 functions. It was found that alteration of Hsp70 phosphorylation status in vivo can impair prion propagation, alter both basal and acquired thermotolerance and in some cases render cells inviable. By looking at analogous mutants in closely related cytosolic Hsp70s this study identified functional similarities and differences between highly homologous Hsp70 species. This study shows a clear link between Hsp70 phosphorylation status and in vivo function. Given Hsp70s central role in a variety of important cellular metabolic pathways and the conservation of these phosphorylatable sites in higher eukaryotes, these findings have far reaching implications