Apparent dominance of serine auxotrophy and the absence of expression of muscle-specific proteins in rat myoblast � mouse L-cell hybrids

Abstract

Hybrids obtained from crosses of rat myoblasts and mouse L-cells are unable to differentiate into myotubes. The synthesis of muscle-specific proteins, creatine phosphokinase (CPK), myosin, and adenyl cyclase is also suppressed in the hybrids. Adenyl cyclase produced in the hybrids is not activated by isoproterenol. The glycine auxotrophy of myoblasts is phenotypically expressed as a dominant trait in hybrids. Genotypically, however, all the enzymes of the phosphorylated pathway of serine biosynthesis and serine hydroxymethyltransferase, the enzyme inter converting serine and glycine, are present in normal concentrations in the hybrids, just as they are in the mouse L-cells. Unlike either of the parental lines, the hybrid cells are unable to grow on serine-supplemented medium, but do so when glycine is also added. Glycine by itself supports growth of the hybrids in the absence of added serine. All the curious effects of glycine/serine on growth of hybrids are rationalized and accounted for by the hypothesis that in these cells the enzyme serine hydroxy-methyltransferase, for unknown reasons, does not function in vivo.