α‐Crystallin Stabilizes Actin Filaments and Prevents Cytochalasin‐Induced Depolymerization in a Phosphorylation‐Dependent Manner

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Abstract
α‐crystallin, a major lens protein of approximately 800 kDa with subunits of about 20 kDa has previously been shown to act as a chaperone protecting other proteins from stress‐induced damage and to share sequence similarity with small heat‐shock proteins, sHsp. It is now demonstrated that this chaperone effect extends to protection of the intracellular matrix component actin. It was found that the powerful depolymerization effect of cytochalasin D could be almost completely blocked by α‐crystallin, αA‐crystallin or αB‐crystallin. However, phosphorylation of α‐crystallin markedly decreased its protective effect. It is suggested that phosphorylation of α‐crystallin may contribute to changes in actin structure observed during cellular remodeling that occurs with the terminal differentiation of a lens epithelial cell to a fiber cell and contributes to cellular remodeling in other cell types that contain α‐crystallin species. This communication presents biochemical evidence clearly demonstrating that α‐crystallin is involved in actin polymerization‐depolymerization dynamics.It is also shown that α‐crystallin prevented heat‐induced aggregation of actin filaments. α‐crystallin was found to stabilize actin polymers decreasing dilution‐induced depolymerization rates up to twofold while slightly decreasing the critical concentration from 0.23 μM to 0.18 μM. Similar results were found with either α‐crystallin or its purified subunits αA‐crystallin and αB‐crystallin. In contrast to the experiments with cytochalasin D, phosphorylation had no effect. There does not appear to be an interaction between α‐crystallin and actin monomers since the effect of α‐crystallin in enhancing actin polymerization does not become apparent until some polymerization has occurred. Examination of the stoichiometry of the α‐crystallin effect indicates that 2‐3 α‐crystallin monomers/actin monomer give maximum actin polymer stabilization.

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