Specific binding of the transglutaminase, platelet factor XIII, to HSP27

HSP27, the unique mammalian low molecular weight heat shock protein, is prominently phosphorylated upon activation of a wide variety of cells and has a role in thermotolerance, growth events, and regulation of actin cytoskeletal dynamics. In thrombin-stimulated platelets, HSP27 is rapidly and prominently phosphorylated in a manner highly correlated with platelet secretion. However, the function of HSP27 and the identity of proteins that interact with HSP27 remain unknown. To identify specific HSP27-protein interactions, a recombinant fusion protein affinity reagent was constructed and used to identify proteins associating with HSP27 from human platelet lysates and erythroleukemia cells. An 84-kDa protein was found to associate specifically with HSP27 and was isolated from platelet lysates, resolved on preparative gels, transferred to nitrocellulose, subjected to enzymatic digestion, and microsequenced. A 20-amino acid sequence derived from p84 proved identical to amino acids 484-503 of the transglutaminase, platelet Factor XIII. Immunoblotting studies were used to confirm the binding of FXIII from fresh platelet lysates to the HSP27 fusion protein. FXIII also was shown to coprecipitate with HSP27 in immunoprecipitation studies and to colocalize with HSP27 in immunofluorescence studies of intact glass-activated platelets. The data thus demonstrate specific binding of platelet FXIII to HSP27 and suggest that HSP27 may participate in the cellular localization and/or enzymatic regulation of platelet FXIII.