TYROSINE PHOSPHORYLATION OF THE DENSE PLAQUE PROTEIN PAXILLIN IS REGULATED DURING SMOOTH-MUSCLE CONTRACTION

Regulation of the attachment of actin filaments to the cell membrane a t membrane-associated dense plaque (MADP) sites could allow smooth mus cle cells to modulate their cytostructure in response to changes in ex ternal stress. In this study, changes in the tyrosine phosphorylation of the MADP protein paxillin were measured by Western blot during the contraction and relaxation of tracheal smooth muscle strips. Tyrosine phosphorylation of paxillin increased by three- to fourfold with a tim e course similar to force development during contractile stimulation w ith acetylcholine (ACh), 5-hydroxytryptamine and KCl and decreased dur ing washout of contractile stimuli and during relaxation induced by fo rskolin. Immunoprecipitation of muscle extracts with multiple rounds o f anti-phosphotyrosine antibody removed similar to 20% of the total pa xillin in resting muscles and similar to 60% of paxillin in muscles ma ximally stimulated with ACh. These results provide the first evidence associating the tyrosine phosphorylation of paxillin with the active c ontraction of smooth muscle or with any functional response of a fully differentiated tissue in vivo. The results are consistent with a role for MADP proteins in the regulation of force development in smooth mu scle.