The small heat shock proteins Hsp20 and αB-crystallin in cultured cardiac myocytes: differences in cellular localization and solubilization after heat stress

Abstract

Hsp20, a recently described new member of the small heat shock protein superfamily, is abundant in heart, skeletal muscle types and smooth muscle. We investigated the intracellular localization of Hsp20 in cultured rat neonatal cardiac myocytes, under normal conditions and after stress. These cellular characteristics of Hsp20 were compared with those of its closest relative, αB-crystallin, which is also highly expressed in heart. Like αB-crystallin, Hsp20 is normally located in the cytoplasm of the cardiac myocytes. After a heat stress, a subpopulation of Hsp20 migrates into the nucleus, while another part remains in the cytoplasm. In very few cells a faint sarcomeric association of Hsp20 is observed. In contrast, as previously reported, αB-crystallin displays a very distinct cross-striated sarcomeric staining after the heat shock, but no nuclear migration. Also at the level of Triton solubility, differences exist between the two related proteins; while αB-crystallin, like other small heat shock proteins, becomes insoluble upon heat stress, Hsp20 remains largely soluble. Our results indicate that Hsp20 and αB-crystallin, despite their structural similarities, display conspicuous functional differences.