Isoforms of the small non-catalytic subunit of smooth muscle myosin light chain phosphatase

Chicken gizzard smooth muscle myosin light chain phosphatase is composed of a similar to 37 kDa catalytic subunit, a similar to 110 kDa myosin binding or targeting subunit and a similar to 20 kDa subunit (MPs) whose function is as yet undefined. It was reported previously that a cloned chicken gizza rd MPs cDNA encodes a protein of 186 amino acids (aa) [Y.H. Chen, M.X. Chen , D.R. Alessi, D.G. Gampbell, C. Shanahan, P. Cohen, P.T.W. Cohen, FEES Let t. 356 (1994) 51-55]. More recently, we obtained by PCR amplification anoth er MPs cDNA that encodes a protein of only 161 aa [Y. Zhang, K. Mabuchi, T. Tao, Biochim. Biophys. Acta 1343 (1997) 51-58]. In this work we obtained c DNAs corresponding to both sequences using a different set of PCR primers, indicating that the two sequences correspond to isoforms that most likely a rose from alternative splicing of the same gene. Using two polyclonal antib odies, one raised against the recombinant 161 aa isoform of chicken gizzard MPs and the other against a C-terminal polypeptide that is present only in the 186 aa isoform, we found that while the 161 aa isoform is the predomin ant one in chicken gizzard, in chicken aorta it is the 186 aa one; in chick en stomach both isoforms are present, and in mammalian tissues such as ferr et and rat only the 186 aa isoform is detected. Furthermore, we purified th e MPs associated with the chicken gizzard myosin light chain phosphatase ho loenzyme and determined its molecular weight, amino acid composition and si x residues of its C-terminal sequence. The results from these analyses show ed conclusively that the predominant isoform in chicken gizzard is the 161 aa one. (C) 1999 Elsevier Science B.V. All rights reserved.