Oligomerization of Hsp70 upon insertion into lipid membranes (785.2)

Abstract

Heat shock proteins (hsp) are well recognized for their protein folding activity. Additionally, hsp expression is enhanced during stress conditions to preserve cellular homeostasis. Hsp are also detected outside cells, released by an active mechanism independent of cell death. Extracellular hsp appear to act as signaling molecules as part of a systemic response to stress. Extracellular hsp do not contain a consensus signal for their secretion via the classical ER‐Golgi compartment. Therefore, they are likely exported by an alternative mechanism requiring translocation across the plasma membrane. We propose that membrane interaction is the first step in the export process of Hsp70, the major inducible hsp, since this protein has been detected on the surface of stressed cells. The question that emerges is how does this highly charged cytosolic protein interact with lipid membranes? Using a liposome insertion assay, we showed that Hsp70 selectively interacted with negatively charged phospholipids, particularly phosphatidyl serine (PS), within liposomes, which was followed by insertion into the lipid bilayer, forming high molecular weight oligomers. Hsp70 displayed a preference for less fluid lipid environments, and the region embedded into the lipid membrane was mapped to the C‐terminus end of the molecule. The results from our studies provide evidence of an unexpected ability of a large, charged protein to become inserted across a lipid membrane. This observation provides a new paradigm for the interaction of proteins with lipid environments. In addition, it may explain the export mechanism of an increasing number of proteins that lack the consensus secretory signals.Grant Funding Source: Supported by NIH R01 GM098455