The small heat shock-related protein, HSP20, is phosphorylated on serine 16 during cyclic nucleotide-dependent relaxation

The small heat shock-related protein 20 (HSP20) is present in four isoforms in bovine carotid artery smooth muscles. Three of the isoforms are phospho rylated and one is not. Increases in the phosphorylation of two isoforms of HSP20 (isoform 3, pI 5.9; and 8, pI 5.7) are associated with cyclic nucleo tide-dependent relaxation of bovine carotid artery smooth muscles. Increase s in the phosphorylation of another isoform (isoform 4, pI 6.0) are associa ted with phorbol ester-induced contraction of bovine carotid artery smooth muscles. In this investigation we determined that isoforms 3 and 8 are phos phorylated on Ser(16) of the HSP20 molecule during activation of cAMP-depen dent signaling pathways. Phosphorylation state-specific antibodies produced against a peptide containing phosphorylated Ser(16) recognized isoforms 3 and 8 but not isoform 4. In human vascular tissue, only isoform 3 is presen t. Incubation of transiently permeabilized strips of bovine carotid artery smooth muscle with synthetic peptides in which Ser(16) is phosphorylated, i nhibits contractile responses to high extracellular KCl and to serotonin, T hese data suggest that phosphorylation of HSP20 on Ser(16) modulates cAMP-d ependent vasorelaxation.