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1 Molecular Pharmacology, Physiology & Biotechnology, Brown University, Providence, RI, USA
* To whom correspondence should be addressed. E-mail: chi-ming_hai{at}brown.edu.
We tested the hypothesis that cholinergic receptor stimulation recruits actin- and integrin-binding proteins from the cytoplasm to the cytoskeletal/membrane complex in intact airway smooth muscle. We stimulated bovine tracheal smooth muscle with carbachol, fractionated the tissue homogenate into pellet (P) and supernatant (S) by ultracentrifugation. In unstimulated tissues, calponin exhibited the highest basal P/S ratio (2.74 ± 0.47), whereas vinculin exhibited the lowest P/S ratio (0.52 ± 0.09). Cholinergic receptor stimulation increased the P/S ratios of the following proteins in descending order of sensitivity: 
-Actinin > Talin ~ Metavinculin > a-SM Actin > Vinculin ~ Calponin. Carbachol induced Erk 1/2 phosphorylation by 300% of basal value. U0126 (10 µM) completely inhibited carbachol-induced Erk 1/2 phosphorylation, but did not significantly affect the correlation between -actinin P/S ratio and [carbachol]. This observation indicates that cytoskeletal/membrane recruitment of -actinin is independent of Erk 1/2 MAPK activation. Metavinculin and vinculin are splice variants of a single gene, but metavinculin P/S ratio was significantly higher than vinculin P/S ratio. Furthermore, the P/S ratio of metavinculin but not vinculin increased significantly in response to cholinergic receptor stimulation. Calponin and a-actinin both belong to the family of Calponin Homology (CH) domain proteins. However, unlike -actinin, the calponin P/S ratio did not change significantly in response to cholinergic receptor stimulation. These findings indicated differential cytoskeletal/membrane recruitment of actin- and integrin-binding proteins in response to cholinergic receptor stimulation in intact airway smooth muscle. -Actinin, talin, and metavinculin appear to be key cytoskeletal proteins involved in the recruitment process.
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