The structures of the M110 and M21 regulatory subunits of protein phosphatase-1M, the major enzyme which dephosphorylates myosin in smooth muscle, have been deduced from cloned cDNAs. The N-terminus of the M110 subunit from rat aorta contains seven ankyrin repeats, while the C-terminus of the M21 subunit from chicken gizzard contains a leucine zipper motif. The M110 subunit is expressed in two different forms which differ in their C-terminal sequences. One of these is highly homologous to the whole of the M21 subunit.
