Synthesis in Cultured Smooth Muscle Cells*

Recently, we have reported on the isolation of a high molecular weight species of soluble elastin from lathyritic chick which was postulated to represent the primary precursor of elastin (proelastin). In order to establish the existence and precursor function of proelastin, second passage rabbit aortic smooth muscle cells, previously shown to be synthesizing insoluble elastin, were pulsed with [$HIvaline and [Wlproline. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the media revealed a prominent high molecular weight protein (120,000 to 130,000) which was labeled with both radioactive proline and valine. This peak was electrophoretically distinct from a slower moving [‘4CIproline-labeled protein chromatographing with a procollagen standard. The high molecular weight protein remained stable in the media for pulse times up to 2 h prior to undergoing degradation to tropoelastin and other, lower molecular weight proteins. In the cell layer, the high molecular weight protein was prominent in short pulse times of 5 and 10 min and was chased into tropoelastin protein within 30 min. Long pulse times of 1 and 2 h revealed that the ratio of the high molecular weight protein to tropoelastin decreased in the cell layer. Evidence that the high molecular weight protein is proelastin was established indirectly by pulse-chase experiments, valine and proline incorporation, mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis relative to procollagen mobility, resistance to collagenase digestion, and ion exchange chromatography and directly by amino acid analysis.