Surface-labelling studies on skeletal-muscle cells in vitro. Heterogeneity of iodinated cell-surface proteins.

1. Two distinct classes of protein were detected at the surface of chick-embryo skeletal-muscle cells after iodination of the cells in monolayer culture. 2. The two classes of iodinated proteins differed in their ability to co-purify with a vesicular plasma-membrane fraction prepared from surface-labelled cells. 3. One class consisted of predominantly high-molecular-weight glycoproteins that co-purified with the plasma-membrane fraction, but showed no significant qualitative or quantitative alterations in labelling with 125I and lactoperoxidase during myogenesis. 4. A second class of predominantly lower-molecular-weight proteins showed reproducible quantitative alterations in 125I-labelling during myogenesis but failed to co-purify with the plasma-membrane fraction. 5. This second class of proteins may represent matrix proteins involved in intercellular adhesion or adhesion of cells to the substratum. They are unlikely to be directly required for the process of plasma-membrane fusion during myogenesis, since they do not copurify with a vesicular plasma-membrane fraction known to be capable of Ca2+-dependent fusion in vitro.