Structural and functional studies on the non-muscle isoform of alpha-actinin.

alpha-Actinin was purified from adult chicken brains, cleaved with thermolysin and analysed by SDS-PAGE. Chicken brain a-actinin gave a similar (but distinct) pattern of thermolytic peptides to the chicken smooth and skeletal muscle a-actinins, suggesting that all three isoforms have similar domain structures. Several polypeptides derived from the actin-binding and repeat domains of chick brain a-actinin were found to be identical in sequence with the corresponding regions of the chick smooth muscle isoform. Indeed, the complete deduced sequence of chick brain a-actinin (893 amino acids) was only divergent with that of the smooth muscle isoform in the region of the N-terminal EF- hand, where 27 residues in brain a-actinin are replaced by 22 residues in smooth muscle a-actinin. These isoforms therefore arise by the alternative splicing of a transcript produced by a single gene. Chick brain a-actinin is predicted to bind two calcium ions per dimer, whereas chick smooth muscle a-actinin is not predicted to bind any. This may explain why chick brain (but not chick smooth muscle) a-actinin binds actin in a calcium- sensitive manner (Duhaiman and Bamburg, 1984). However, in this analysis chick brain a-actinin was not found to bind calcium using two different in-vitro assay procedures. Both chick brain and smooth muscle isoforms became incorporated into stress-fibres and cell-matrix junctions when complete cDNAs were expressed in monkey COS cells, suggesting that these isoforms are not differentially localised in non-muscle cells. The complete cDNA sequence of a human placental a-actinin was found to display a high level of identity with the chick brain sequence across the transcribed regions (85 and 97% identity at the DNA and protein levels respectively).