myotonin-protein kinase is a serine-threonine kinase that is closely related to other kinases that interact with members of the rho family of small gtpases. substrates for this enzyme include myogenin, the beta-subunit of the l-type calcium channels, and phospholemman. although the specific function of this protein is unknown, it appears to play an important role in muscle, heart, and brain cells. this protein may be involved in communication within cells. it also appears to regulate the production and function of important structures inside muscle cells by interacting with other proteins. for example, myotonic dystrophy protein kinase has been shown to turn off (inhibit) part of a muscle protein called myosin phosphatase. myosin phosphatase is an enzyme that plays a role in muscle tensing (contraction) and relaxation.