Plasma Membrane Protein Subunit Composition

The protein subunit composition of three well defined plasma membrane fractions from rat liver, kidney, and erythrocyte, liver and kidney mitochondria, and liver smooth endoplasmic reticulum has been compared by high resolution sodium dodecyl sulfate (SDS) disc gel electrophoresis. Membranes are completely solubilized in SDS, and the proteins are reduced to subunits, fractionated according to subunit size, and stained. Nonidentical subunits can be distinguished from subunits having identical sizes by electrophoretic mixing experiments with the use of two different membranes. In each membrane, 35 to 40 bands are resolved, and a correlation is found between the reciprocal of the number of nonidentical bands and the functional interrelatedness of the membranes. No subunit accounting for more than 1% of the total membrane protein is common to each membrane. However, the plasma membranes share a major band of identical relative mobility having an estimated molecular weight of 48,000. These subunits are indistinguishable when electrophoresed in the absence of SDS and are probably identical.