Differential calmodulin binding to three myosin-1 isoforms from liver.

We have previously purified and characterized two myosin-1 isoforms from rat liver (molecular masses 130 kDa and 110 kDa; L. M. Coluccio and C. Conaty (1993) Cell Motil. Cytoskel. 24, 189-199). Here, we describe the purification and characterization from liver of a third myosin-1 (molecular mass 105 kDa) and determine the number of calmodulin molecules associated with each of these three myosin-1 isoforms. The 105 kDa polypeptide, solubilized from liver homogenates with the addition of ATP, co-sediments with F-actin, co-purifies with calmodulin, and binds calmodulin in the presence of EGTA. Antibodies directed against chicken intestinal brush border myosin-1 cross-react with the 105 kDa polypeptide on immunoblots. Partial peptide sequence analysis indicates that the polypeptide corresponds with an MM1 gamma gene product that represents a myosin-1 isoform cloned from mouse brain (Sherr et al. (1993) J. Cell Biol. 120, 1405-1416). A comparison of calmodulin binding to the now three isolated forms of myosin-1 in liver shows that in solution the 105 kDa and 110 kDa polypeptides bind two molecules of calmodulin each whereas the 130 kDa binds six molecules of calmodulin.