HSP70 Associates with Phosphatidylserine Membranes via the Peptide Binding Domain

Abstract

The expression of heat shock proteins (hsp) is a natural response to an array of physiological, environmental and clinical stressors. These proteins participate in the repair and recovery from an insult and confer protection from subsequent stresses. The cytoprotective effect of hsp has been associated with their chaperone function within the cytosol. Recently, they have been found in the extracellular environment where they propagate the stress signal to avoid the dissemination of the insult. Hsp70, the major inducible form of the hsp family, does not contain any consensus secretory signal that predicts its export via the classical ER-Golgi secretory pathway. We have proposed that Hsp70 is exported by a novel mechanism that is initiated by the translocation of the protein into the plasma membrane. Furthermore, the protein is released associated with extracellular vesicles (ECV), which we speculate result in a robust activation of the immune system. We investigated the mechanism of Hsp70 insertion into membranes using liposomes. We observed that Hsp70 insertion into lipid membranes was spontaneous and specific for negatively charged lipids, such as phosphatidylserine. Using a proteomic approach, we determined that the C-terminus end of the molecule, which contains the peptide binding domain, is inserted into the lipid bilayer. The N-terminus of the molecule, containing the ATP binding site, is exposed to the external part of the liposome. These results resemble our initial studies characterizing channel activity of Hsp70 observed in artificial lipid membranes.Supported by NIH R01GM098455.