The Small Heat Shock Proteins, Hspb1 And Hspb6, Have The Ability To Get Inserted Into Lipid Membranes

Increasing evidence has indicated that heat shock proteins (HSP) escape their cytosolic environment acting as signaling agents in the extracellular fluid. Since the majority of these proteins lack the information necessary for their export via the classical secretory pathways, special attention has been directed at alternative mechanisms for their release. In this regard, a major obstacle for their secretion is the crossing of the plasma membrane. A mechanism that appears equally suitable for their release is via exosomes. Indeed, several hsp have been detected within exosomes isolated from various sources. Herewith we report that a member of the small hsp family, HSPB1 (Hsp27), was found to be associated with exosomes and apparently inserted in the vesicle membrane. In order to investigate the potential interaction of HSPB1 and lipid membranes, pure protein was incubated with liposomes made of various lipid compositions and insertion was accessed by high-speed centrifugation. We found that HSPB1 interacts with liposomes made of palmitoyl oleoyl phosphatidylserine (POPS), phosphatidylcholine (POPC) and phosphatidylglycerol (POPG), however, with different characteristics. Another member of the small HSP family, alphaB-crystallin (HSPB5), also interacts with liposomes, but differently than HSPB1. To address the regions interacting with the membrane, intact proteoliposomes were digested with proteinase K and the protected domains within the liposomes were detected by mass spectroscopy. We observed that large parts of HSPB1 and HSPB6 were embedded within the liposomes. These observations suggest that the interaction with lipid membranes may be part of the mechanisms of export of these proteins.