CLONING OF THE CDNA-ENCODING RAT MYOSIN HEAVY CHAIN-A AND EVIDENCE FOR THE ABSENCE OF MYOSIN HEAVY CHAIN-B IN CULTURED RAT MAST (RBL-2H3) CELLS

The complete amino acid sequence (1961 amino acids) of a vertebrate ce llular myosin heavy chain-A was deduced from cDNA clones of a secretor y rat mast cell line, the RBL-2H3 cell. The rat, human and chicken cel lular myosin heavy chain-A exhibited high similarity in domains that a llow binding of ATP and actin. The amino acid sequence of non-muscle m yosin heavy chain-A from rat was 96% identical to that in human and 99 % identical to that in chicken. Northern blot analysis of mRNA indicat ed the presence of single message of 7.4 kilobases. Northern blot, rev erse-transcriptase polymerase chain reaction, and Western blot with is oform-specific antibodies indicated that RBL-2H3 cells expressed exclu sively myosin heavy chain-A. Unlike rat PCl2 cells, as well as a wide variety of other cultured cells and tissues, myosin heavy chain-B mRNA and protein were not detectable in RBL-2H3 cells. Because RBL-2H3 cel ls can be stimulated to release secretory granules as well as newly ge nerated arachidonic acid and cytokines but lack myosin heavy chain-B, this cell line may provide a unique model to study the role of myosin heavy chain-A in cellular responses to antigen and other stimulants.