A chick skeletal‐muscle α‐actinin gene gives rise to two alternatively spliced isoforms which differ in the EF‐hand Ca2+‐binding domain

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Abstract
A chick non‐muscle α‐actinin cDNA probe encoding the EF‐hand region of molecule was used to screen a λgt10 chick brain cDNA library from 14‐day embryos. A partial 2.1‐kb α‐actinin cDNA was isolated (8W cDNA) which encoded a protein identical to chick skeletal‐muscle α‐actinin, except in the C‐terminal part of the first EF hand. In the variant, the 22 residues found in the skeletal‐muscle isoform were replaced by a stretch of 26 unique residues. Analysis of the structure of the skeletal‐muscle α‐actinin gene showed that the region of divergence was encoded by two exons which are alternatively spliced. Quantitative reverse transcriptase/polymerase chain reaction (RT/PCR) was used to investigate the levels of the α‐actinin transcripts in various tissues. The skeletal‐muscle α‐actinin variant was expressed at low levels in brain, liver and spleen, but could not be detected in skeletal muscle. Surprisingly, skeletal‐muscle α‐actinin mRNA was also expressed in brain, liver and spleen. The RT/PCR products were authenticated by using diagnostic restriction enzyme sites and by sequencing. The splice variant derived from the skeletal‐muscle α‐actinin gene was also detected in a variety of cDNA libraries from both adult and embryonic tissues by PCR. Although a transcript encoding this α‐actinin splice variant is expressed in non‐muscle tissues, neither of the two EF‐hands would be predicted to be functional, making it unlikely to be a typical non‐muscle isoform which are calcium‐sensitive with respect to binding actin. The two vertebrate non‐muscle α‐actinins sequenced to date also have a spacer of five amino acids between the two EF hands, whereas in the variant, the spacer is just four residues in length. Further analysis will be required before this α‐actinin isoform, which we refer to as SKv, can be classified as muscle or non‐muscle α‐actinin. We propose a new nomenclature to describe the various α‐actinin genes and their transcripts.