The small heat shock-related protein-20 is an actin-associated protein

Purpose: The activation of cyclic nucleotide-dependent signaling pathways in vascular smooth muscle is important for the prevention of vein graft spasm and neointimal hyperplasia. Cyclic nucleotide-dependent relaxation is associated with an increase in the phosphorylation of a small heat shock-related protein (HSP20). In this investigation, we examined the mechanisms by which HSP20 may modulate relaxation. Methods: The relaxation responses of the bovine carotid artery smooth muscles were determined in a muscle bath. HSP20 phosphorylation was quantitated with isoelectric-focusing immunoblots. The association with actin was determined with coimmunoprecipitation and cosedimentation. Molecular sieving columns were used to examine the macromolecular associations of HSP20. Results: The activation of cyclic nucleotide signaling pathways leads to the complete relaxation of carotid smooth muscle. This relaxation response is associated with an increase in the phosphorylation of HSP20. Actin coimmunoprecipitated with HSP20, and the association of actin with recombinant HSP20 in vitro was phosphorylation-state dependent. Finally, HSP20 exists in large (>100 kDa) aggregates, which dissociate with the activation of cyclic nucleotide signaling pathways. Conclusion: These data support a role of HSP20 phosphorylation in mediating smooch muscle relaxation, possibly via a direct interaction of large aggregates of HSP20 with the contractile elements.