Overexpression of sTnC polypeptide in muscle cells is controlled by its rapid degradation

The check-points that maintain stoichiometric synthesis of muscle proteins were examined by misexpression of slow troponin C (sTnC) in mouse C2 myotubes. The sTnC mRNA was overexpressed in myotubes by transfecting these cells with a plasmid construct containing the constitutive CMV promoter-driven sTnC cDNA. An approximately four-fold increase of sTnC mRNA level in the transfected cells was observed. However, the increased mRNA level did not produce a corresponding increase of the sTnC polypeptide level in transfected cells. Only a modest 1.5-fold increase of the sTnC polypeptide level in the transfected cells was observed. The excess sTnC polypeptide in transfected cells was found in the soluble form which was not complexed with other thin filament proteins. The difference between the increase of sTnC mRNA and the polypeptide levels in transfected cells was not due to inefficient translation of the overexpressed sTnC mRNA. Analyses of the stability of the sTnC polypeptide in the thin filament and in the unassembled soluble forms showed that the excess soluble sTnC polypeptide was degraded more rapidly than the sTnC polypeptide of the thin filament. Analyses of the mRNA and polypeptide levels of several thin filament complements showed no effect of overexpression of the sTnC mRNA.