Cyclic AMP-stimulated phosphorylation of bovine tracheal smooth muscle contractile and non-contractile proteins.

1. Various proteins isolated from bovine tracheal smooth muscle were examined as phosphate acceptor substrates for a cyclic AMP-dependent protein kinase isolated from the same tissue. A fraction prepared in a manner similar to that of skeletal muscle troponin was the best substrate of the presumptive contractile proteins isolate. Actomyosin and tropomyosin were relatively poor substrates. 2. An assay was developed for the rapid detection in a large number of samples of the muscle specific substrate for the protein kinase on which we reported previously. 3. Using this assay, the muscle specific substrate found in bovine tracheal smooth muscle was partially purified resulting in a preparation which when resolved by polyacrylamide gel electrophoresis showed a single peak of 32P incorporated, and which could be further characterized. 4. Our findings suggest that the substrate contains a protein subunit of molecular weight 19 000, which can be phosphorylated at serine and threonine residues, in the presence of cyclic AMP and protein kinase. The phosphate is in a covalent ester linkage with these residues. 5. A phosphoprotein phosphatase was isolated from the bovine tracheal smooth muscle. 6. Bovine tracheal smooth muscle contains cyclic AMP dependent protein kinase and phosphoprotein phospahatase activity as well as the muscle specific substrate, suggesting that these elements may be part of a mechanism which regulates smooth muscle tone.