Protein and Glycoprotein Subunit Composition of Plasma Membrane from Chick Embryo Fibroblasts

Abstract

Plasma membranes, isolated from uninfected cultures of chick embryo fibroblasts, were dissociated with sodium dodecyl sulfate and analyzed for glycoprotein and protein subunit composition by discontinuous gel electrophoresis. More than 30 protein subunits and 18 glycoprotein subunits were obtained from the A′ and B′ plasma membrane fractions; the membranes in the A′ fraction contain almost twice as much phospholipid and cholesterol as those of the B′ fraction and have a density of 1.06 versus 1.12. Electrophoretic separation profiles were similar for both types of membranes. The molecular weights of the components range from more than 220,000 to less than 22,500. Major protein subunits had molecular weights of 175,000, 95,000 75,000, 69,000, 56,000, 47,000 35,000, and 32,000. The majority of the glycoprotein subunits migrated with protein subunits of more than 70,000 molecular weight. No estimation of molecular weights was attempted for these components. Iodination of whole cells yielded A′ and B′ membrane fractions, which were differentially labeled, with five and seven peaks of radioactivity, respectively. Some degree of differentiation between A′ and B′ fractions was also obtained when cells were prelabeled with radioactive glucosamine and mannosamine. A′ membrane fractions, known to contain more lipid than B′ membrane, also showed a greater recovery of isotope in the lipid region of the gel than did B′ membrane.