Alpha B-crystallin in cardiac tissue. Association with actin and desmin filaments.

alpha B-Crystallin is a 20-kd peptide highly homologous to the small heat-shock proteins. This protein forms soluble homomultimeric complexes (M(r), 300-700 kd) and is very abundant in cardiac muscle cells. In vitro experiments (affinity column chromatography and binding studies with isolated proteins) have shown that alpha B-crystallin interacts directly with actin and, in particular, with desmin filaments. The immunocytochemical localization of alpha B-crystallin within the cardiomyocytes showed that the protein is distributed exclusively in the central region of the I bands (Z lines), where desmin is localized. In vitro studies have further shown that the binding affinity of alpha B-crystallin to actin and desmin filaments increases considerably at slightly acidic pH (6.5) or after a heat treatment (45 degrees C). Moreover, alpha B-crystallin was found to prevent effectively the tendency of actin filaments to form aggregates (i.e., paracrystals) at acidic pH. These in vitro data suggest a protective role of alpha B-crystallin during stress conditions such as ischemia of the heart. Crystallin could prevent the aggregation of filaments, which might occur during the acidification of the cytosol and lead eventually to irreversible structural damage.