Molecular cloning of low‐Ca2+‐sensitive‐type non‐muscle α‐actinin

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Abstract
We previously reported the purification and characterization of a novel non-muscle α-actinin from chicken lung [Imamura, M. & Masaki, T. (1992) J. Biol. Chem. 267, 25927–25933]. The Ca2+ sensitivity of the lung α-actinin for the interaction with polymerized actin (F-actin) was much lower than those of the other reported non-muscle α-actinins. Here, we isolated a cDNA clone encoding the novel α-actinin by screening a chicken lung γgt11 cDNA library with antibody specific for the low-Ca2+-sensitive α-actinin. The deduced amino acid sequence of the lung α-actinin showed 76%, 82% and 83% identity to those of chicken skeletal-muscle, smooth-muscle and fibroblast-type α-actinin, respectively. Marked difference in the structure between the lung-type and the other α-actinins was found in the extreme NH2-terminal and in the COOH-terminal half; in the third and fourth regions of four spectrin-like repeats, and in two Ca2+-binding EF-hand consensus regions. The NH2-terminal-side EF-hand contained a notable defect in one of the five oxygen-containing amino acid side chains involved in chelating Ca2+, suggesting that the lower Ca2+ sensitivity of the lung α-actinin is ascribable to this defect. Northern blot analysis showed that the expression pattern of lung-type α-actinin mRNA in various non-muscle tissues differed from that of the other known non-muscle-type (fibroblast-type) α-actinin. The present results clearly demonstrate the existence of two structurally and functionally different types of non-muscle α-actinin; high-Ca2+-sensitive-type (NM1) and low-Ca2+-sensitive-type (NM2) α-actinin.