In contrast to fast and slow skeletal muscle cells that fuse and terminally differentiate, smooth muscle cells are able to simultaneously proliferate and express lineage-restricted proteins. One of these proteins, expressed exclusively in smooth muscles, has been referred to as SM22-alpha, a 22 kDa protein with structural similarity to the vertebrate thin filament myofibrillar regulatory protein calponin and the Drosophila muscle protein mp20, neither of which play a direct role in the contractile apparatus.The protein is a transformation and shape-change sensitive actin cross-linking/geling protein found in fibroblasts and smooth muscle. Its expression is down-regulated in many cell lines, and this down-regulation may be an early and sensitive marker for the onset of transformation. A functional role of this protein is unclear.
