Tissue Specificity and Localization

The results of immunoblotting with antibodies against CP indicate that CP is more or less exclusively expressed in smooth muscle in vivo (Takahashi et al., 1987 Gimona et al., 1990). It is not found in skeletal or cardiac muscle and is absent in nonmuscle tissues, including brain (Takahashi et al., 1987), kidney, liver, and spleen (Gimona et al., 1990). But there are detectable amounts in blood platelets (Takeuchi et al., 1991), cells that express other proteins typical for smooth muscle (Turner and Burridge, 1989), and it has also been reported to occur in adrenal medulla (Takahashi etal., 1987). In the latter case, however, the investment of vascular tissue makes it difficult to exclude the possibility of the presence of contaminating smooth muscle. 

Studies using immunofluorescence microscopy of tissue sections and cultured cells confirm the immunoblotting data and reveal additional aspects of CP distribution. During development in the chick, CP is exclusively restricted to smooth muscle and its putative precursor cells (Duband et al., 1993). In humans it is also found in myoepithelial cells and in myofibroblasts of mammary tissue (hazard et al., 1993). Both of these cell types express other smooth muscle markers, including smooth muscle actin and, variably, smooth muscle myosin (hazard et al., 1993), and have 

Weak labeling for CP has been reported on the actin bundles of fibroblasts (Takeuchi et al, 1991), but the antibody used in this study cross-reacted also with a 23-kDa polypeptide, which could correspond to the sequence-related smooth muscle protein SM22 that is not down-regulated in culture (Gimona et al, 1990) and has been described in primary fibroblasts (San-tarfen et al, 1987). 

In the meantime, the ability to generate large quantities of purified recombinant CP and its subdomains opens the way for the production of crystals for the determination of the molecular structure via X-ray crystallography. This should provide the basis for analyzing in more detail the interaction between CP and its binding partners. We can also anticipate more insight into the in vitro function of CP subdomains from binding studies and in vitro motility assays. 

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