To characterize the in situ interactions between the subunits (regulatory 110 kDa, M110; 21-kDa, M21 and catalytic, 37-kDa, PP1(C)) of smooth muscle myosin phosphatase (SMPP-1M), we determined, in Triton-X-100-permeabilized rabbit portal vein contracted with microcystin-LR, the ability of the following fragments of M110 to regulate relaxation induced by exogenous PP1(C) (a) M110 purified from pig bladder; (b) the 72.5-kDa N-terminal fragment expressed from rat kidney cDNA [glutathione-S-transferase-M110-(11-612)-peptide]; (c) a 58-kDa fragment, the N-terminal degradation product of M110 (M58); (d) two fragments expressed from rat aorta cDNA [M110-(1-309)-peptide and M110-(39-309)-peptide]; a synthetic fragment of M110 [M110-(1-38)-peptide]. The M110/M21 complex accelerated approximately 1.6-fold the rate of dephosphorylation of the myosin P-light chain and also relaxation induced by PP1(C). The glutathione-S-transferase-M110-(11-612)-peptide and the M58 fragments, as well as the M110-(1-309)-peptide and, at higher concentration, M110-(1-38)-peptide, had similar effects that did not require the M21 subunit. Arachidonic acid, known to dissociate PP1(C) from the native holoenzyme and inhibit SIMPP-1M activity, inhibited the regulatory action of the M110/M21 complex on PP1(C) activity and, to a lesser extent that, of the glutathione-S-transferase-M110-(11-612)-peptide, but not that of the M58 fragment or of the shorter peptides. We conclude that, consistent with in vitro studies, the N-terminal sequence (1-309) of the M110 subunit is also sufficient to enhance the activity of PP1(C) for myosin in muscle. However, its C-terminal half (downstream from the M58 fragment) is required for inhibition by arachidonic acid. In contrast to the effect of the M110 subunit and its fragments, a peptide, corresponding to part of the PP1(C)-binding site of the regulatory glycogen-binding subunit from skeletal muscle G(M) [G(M)-(63-93)-peptide], specifically slowed the relaxation, induced by flash photolysis of diazo-2, of Triton X-100-permeabilized femoral artery strips, and inhibited the holoenzyme-induced relaxation in the portal vein, suggesting that the G(M) subunit can compete with the regulatory effect of M110 on PP1(C) in smooth muscle.
- Arachidonic acid
- Calcium senstization
- Protein phosphatase
- Smooth muscle
- Smooth muscle myosin phosphatase