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 mumol/L TAT-MYPT1(1-374), 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 Ca2+ under 118 mmol/L K+ depolarization, with no augmentation of the [Ca2+](i) elevation. The deletion of the Tat peptide, MYPT1(1-374), abolished the augmenting effect. TAT-MYPT1(1-296) demonstrated a weaker but significant augmentation (1.7-fold). However, TAT-MYPT1(1-171), TAT-MYPT1(39- 374), TAT-MYPT1(39 - 296), and TAT-MYPT1(297 - 374) had no augmenting activity. The myosin light chain phosphorylation level as a function of extracellular Ca2+ concentrations was shifted to the left in the strips pretreated with TAT-MYPT1(1-374) 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.