Calponin and mitogen-activated protein kinase signaling in differentiated vascular smooth muscle

Contraction of smooth muscle cells is generally assumed to require Ca2+/calmodulin-dependent phosphorylation of the 20-kDa myosin light chains. However, we report here that in the absence of extracellular calcium, phenylephrine induces a contraction of freshly isolated ferret aorta cells in the absence of increases in intracellular ionized calcium or light chain phosphorylation levels but in the presence of activation of mitogen-activated protein kinase. A protein at 36 kDa co-immunoprecipitated with the mitogen-activated protein kinase and was identified as the actin-binding protein, calponin, by immunoblot. An overlay assay further confirmed an interaction between the kinase and calponin, even though the kinase did not phosphorylate calponin in vitro. Calponin also co-immunoprecipitated from smooth muscle cells with protein kinase C-epsilon. High resolution digital confocal studies indicated that calponin redistributes to the cell membrane during phenylephrine stimulation at a time when mitogen-activated protein kinase and protein kinase C-epsilon are targeted to the plasmalemma. These results suggest a role for calponin as a signaling molecule, possibly an adapter protein, linking the targeting of mitogen-activated protein kinase and protein kinase C-epsilon to the surface membrane.