PHI-1 interacts with the catalytic subunit of myosin light chain phosphatase to produce a Ca²⁺ independent increase in MLC₂₀ phosphorylation and force in avian smooth muscle

In avian smooth muscles, GTPγS produces a Rho kinase mediated increase in PHI-1 phosphorylation and force, but whether this correlation is causal is unknown. We examined the effect of phosphorylated PHI-1 (P-PHI-1) on force and myosin light chain (MLC₂₀) phosphorylation at a constant [Ca²⁺]. P-PHI-1, but not PHI-1, increased MLC₂₀ phosphorylation and force, and phosphorylation of PHI-1 increased the interaction of PHI-1 with PP1c. Microcystin induced a dose-dependent reduction in the binding of PHI-1 to PP1c. These results suggest PHI-1 inhibits myosin light chain phosphatase by interacting with the active site of PP1c to produce a Ca²⁺ independent increase in MLC₂₀ phosphorylation and force.