The exogenously added small subunit of smooth muscle myosin phosphatase increases the Ca2+ sensitivity of the contractile apparatus in the permeabilized porcine renal artery

The effects of the small noncatalytic subunit of myosin light chain phospha tase (MLCPsr) on the Ca2+-induced contraction of smooth muscle were investi gated in the Triton X-100-permeabilized porcine renal artery. The full-leng th recombinant chicken MLCPsr obtained by the bacterial expression system i nduced an additional contraction at a constant [Ca2+](i) and shifted the [C a2+](i)-force relation curve to the left. A deletion mutant containing the N-terminal 78 amino acids of MLCPsr retained the full action, compared with the full-length MLCPsr, while the deletion of this region completely aboli shed its effect. The process of relaxation was also delayed by the fragment containing the N-terminal 78 amino acids. These results indicated that MLC Psr increases the Ca2+ sensitivity of the contractile apparatus while the N -terminal 78 amino acids are responsible for this effect in vascular smooth muscle. (C) 1999 Academic Press.