Size distribution of membrane proteins of rat liver and their relative rates of degradation.

The proteins associated with various membrane fractions, including smooth and rough endoplasmic reticulum, plasma membrane, and mitochondria of rat liver, were solubilized with sodium lauryl sulfate and separated by electrophoresis on sodium lauryl sulfate acrylamide gels. Smooth and rough endoplasmic reticulum have similar electrophoretic patterns. All other fractions have characteristic patterns in which no single or predominant protein occurs. The smooth endoplasmic reticulum fraction was fractionated by use of Triton X-100, which solubilized approximately 50% of the membrane proteins. The patterns of Triton X-100~soluble and -insoluble proteins are distinctive. The relative rates of degradation of the proteins were studied by use of a double isotope technique in which 14Cleucine was administered to intact rats 4 days prior to administration of aH-leucine. As indicated by the differences in 3H:14C ratios, there is a marked heterogeneity of turnover of proteins of both the endoplasmic reticulum and plasma membrane. There is a general correlation between the relative degradation rates of membrane proteins and the molecular size of the protein (subunit) as separated by electrophoresis in sodium lauryl sulfate acrylamide gels. This general relationship was found also for the soluble protein fraction of rat liver, whether fractionated by sodium lauryl sulfate acrylamide gels, or by Sephadex G-ZOO columns in the presence of sodium lauryl sulfate. A model in which membrane proteins associate and dissociate from the protein-lipid complex is favored to explain the heterogeneity of degradation, and the general correlation of size with degradation rate.