Molecular cloning of low-Ca(2+)-sensitive-type non-muscle alpha-actinin.

We previously reported the purification and characterization of a novel non-muscle alpha-actinin from chicken lung [Imamura, M. & Masaki, T, (1992) J. Biol. Chem. 267, 25927-25933]. The Ca2+ sensitivity of the lung alpha-actinin for the interaction with polymerized actin (F-actin) was much lower than those of the other reported non-muscle alpha-actinins. Here, we isolated a cDNA clone encoding the novel alpha-actinin by screening a chicken lung lambda g11 cDNA library with antibody specific for the low-Ca(2+)-sensitive alpha-actinin. The deduced amino acid sequence of the lung alpha-actinin showed 76%, 82% and 83% identity to those of chicken skeletal muscle, smooth-muscle and fibroblast-type alpha-actinin, respectively. Marked difference in the structure between the lung-type and the other alpha-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 Ca(2+)-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 alpha-actinin is ascribable to this defect. Northern blot analysis showed that the expression pattern of lung-type alpha-actinin mRNA in various non-muscle tissues differed from that of the other known non-muscle-type (fibroblast-type) alpha-actinin. The present results clearly demonstrate the existence of two structurally and functionally different types of non-muscle alpha-actinin; high-Ca(2+)-sensitive-type (NM1) and low-Ca(2+)-sensitive-type (NM2) alpha-actinin.

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