Transduction of the N-Terminal Fragments of MYPT1 Enhances Myofilament Ca2+ Sensitivity in an Intact Coronary Artery

OBJECTIVE—The region of the 110 kDa regulatory subunit (MYPT1) of smooth muscle myosin phosphatase involved in the regulation of contraction was determined under physiological conditions. METHODS AND RESULTS—Using HIV Tat protein-mediated protein transduction, the N-terminal fragments of MYPT1 were introduced to the intact porcine coronary arterial strips. Pre-incubation with 3 μmol/L TAT-MYPT1, a construct containing the Tat peptide and the residues 1 to 374 of MYPT1, for 15 minutes augmented (2.4-fold) the subsequent contraction induced by adding 1.25 mmol/L of extracellular Ca under 118 mmol/L K depolarization, with no augmentation of the [Ca]i elevation. The deletion of the Tat peptide, MYPT1, abolished the augmenting effect. TAT-MYPT1 demonstrated a weaker but significant augmentation (1.7-fold). However, TAT-MYPT1, TAT-MYPT1, TAT-MYPT1, and TAT-MYPT1 had no augmenting activity. The myosin light chain phosphorylation level as a function of extracellular Ca concentrations was shifted to the left in the strips pretreated with TAT-MYPT1 compared with the control. CONCLUSIONS—Region 1 to 296 was the minimal region involved in the enhancement of contraction, and region 297 to 374 played a supplemental role. These results suggested that the interaction mainly between catalytic subunit and MYPT1 play a critical role in the regulation of the endogenous myosin phosphatase in intact smooth muscle.