CHARACTERIZATION OF A NOVEL COFILIN ISOFORM THAT IS PREDOMINANTLY EXPRESSED IN MAMMALIAN SKELETAL-MUSCLE

Cofilin is an actin-modulating protein of 20 kDa, which is widely dist ributed throughout muscle and nonmuscle cells. By means of immunoblott ing combined with two-dimensional gel electrophoresis, we found that t wo cofilin variants, muscle type (M-type) and nonmuscle type (NM-type) , exist in mammals, while a single isoform exists in chickens. During in vitro myogenesis of mouse C2 cells, expression of the M-type cofili n was upregulated. To better understand the nature of the M-type cofil in, we cloned cDNAs encoding M-type cofilin from the cDNA library of C 2 myotubes and determined the entire sequence. The deduced peptide seq uence contained a nuclear localization signal and a putative actin-bin ding sequence as reported in NM-type cofilin. The sequence showed 81% identity in the amino acid residues with the mouse NM-type cofilin seq uence and, interestingly, higher homology (96% identity) with that of chicken cofilin. The mRNA encoding M-type cofilin, though it contains two variants that differ in the size of their 3'-non coding sequences, was detected predominantly in heart, skeletal muscle, C2 myotubes, an d testis by Northern blotting, while the mRNA for NM-type cofilin was seen in a variety of non-muscle tissues. The presence of the muscle ty pe isoform of cofilin strongly suggests that cofilin is deeply involve d in the regulation of actin function not only in non-muscle cells but also in muscle cells.