Interactions of protein phosphatase type 1, with a focus on myosin phosphatase

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Abstract
It has been established for many years that MLCK is regulated by the intracellular Ca2+ concentration via the formation of the Ca2+-calmodulin-MLCK complex. A more recent discovery has been that the myosin phosphatase may also be regulated. This is manifest at suboptimal Ca2+ levels where under certain conditions (e.g. stimulation with several agonists) the MP is inhibited. The net result being that the extent of myosin phosphorylation for a fixed Ca2+ level is increased, i.e. an enhanced Ca2+-sensitivity. Spurred by this intriguing discovery several laboratories began studies on MP with an emphasis to determine the regulatory, or inhibitory, mechanism. A similar preparation was obtained by 3 laboratories and consisted of a catalytic subunit, PPlδ, plus a large subunit (M130/133 for gizzard, M130 for bladder and M 110 for rat aorta) and a smaller subunit of 20–21 kD. The isolated catalytic subunit has a much lower activity towards phosphorylated myosin than the holoenzyme, thus the non-catalytic subunits may serve as targeting proteins and in addition may play a regulatory role. Because of the difference in activities between the catalytic subunit and holoenzyme, one mechanism of regulation may involve dissociation of the trimeric complex, and such was proposed for the effect of arachidonic acid. Another suggested regulatory mechanism was that phosphorylation of the large subunit in its C-terminal half caused inhibition of phosphatase activity. The two mechanisms need not be mutually exclusive and in addition several kinases could influence the activity of the myosin phosphatase. In order to understand the molecular basis of phosphatase regulation it is necessary to determine the topography of the holoenzyme and identify sites of interaction between subunits and substrate. This work is in progress. Using various truncation mutants of M130/133 it has been determined that the binding sites for both PPlc and substrate are located within the N-terminal part of the molecule. The M20 subunit binds to the C-terminal end, although the functional significance of this is not established. Many questions remain to be answered concerning the biochemistry of the myosin phosphatase. An exciting and challenging focus will be to determine the mechanism(s) of regulation and to unravel the signaling cascade(s) that are initiated by agonist-receptor complex formation. In addition, the location of the MP is not known and it is important to establish which (if any) of the cytoskeletal elements are involved in binding to MP. Finally, it is assumed that the trimeric phosphatase, as discussed above, is specific for myosin dephosphorylation and does not act on other substrates. Because of the breadth of its distribution in different tissues and the wide range of proteins interacting with the ankyrin repeats it is possible that this phosphatase, or variants thereof, has roles in other cellular processes. (Mol Cell Biochem 190: 79–84, 1999)