Defined sequence segments of the small heat shock proteins HSP25 and alphaB-crystallin inhibit actin polymerization

The interaction of small heat shock proteins (sHSPs) with the actin cytoske leton has been described and some members of this family, e.g. chicken and murine HSP25 (HSP27), inhibit the polymerization of actin in vitro. To anal yse the molecular basis of this interaction, we synthesized a set of overla pping peptides covering the complete sequence of murine HSP25 and tested th e effect of these peptides on actin polymerization in vitro by fluorescence spectroscopy and electron microscopy. Two peptides comprising the sequence s W43 to R57 (peptide 6) and I92 to N106 (peptide 11) of HSP25 were found t o be potent inhibitors of actin polymerization. Phosphorylation of N-termin ally extended peptide 11 at serine residues known to be phosphorylated in v ivo resulted in decline of their inhibitory activity. Interestingly, peptid es derived from the homologous peptide 11 sequence of murine alphaB-crystal lin showed the same behaviour. The results suggest that both HSP25 and alph aB-crystallin have the potential to inhibit actin polymerization and that t his activity is regulated by phosphorylation.