Structure and sequence of the myosin alkali light chain gene expressed in adult cardiac atria and fetal striated muscle

Mammalian cardiac muscle contains two myosin alkali light chains which are the major isoforms present in either atrial (MLC1(A)) or ventricular (MLC1(V)) muscle, and which are different from the fast skeletal muscle isoforms (MLD1(F) and MLC3(F)). The atrial isoform is also expressed in fetal skeletal and fetal ventricular muscle, where this isoform is also described as the fetal isoform MLC1emb. We have previously isolated a cDNA clone encoding part of the mouse MLC1(A)/MLC1emb isoform and have used this clone to demonstrate the identity of MLC1(A) and MLC1emb in the mouse. To date no information on the amino acid sequence of this mammalian atrial/fetal isoform has been available. Here we present the complete structure and sequence of the mouse MLC1(A)/MLC1emb gene, together with the predicted amino acid sequence of this isoform. Comparison of the MLC1(A)/MLC1emb gene and polypeptide with those of MLC1(F) and MLC1(V) suggests that MLC1(A)/MLC1emb and MLC1(V) were generated from a common ancestral gene. The NH₂-terminal region of MLC1(A)/MLC1emb, thought to be involved in the actomyosin interaction, shows conservation with MLC1(V) but not with MLC1(F) suggesting a shared functional domain in these cardiac isoforms. Comparison with the chicken embryonic MLD (L₂₃) suggests that although MLC1(A)/MLC1emb and L₂₃ show very different patterns of expression, both during development and in the adult, they probably represent the homologous gene in these two species.