The turnover of cell surface proteins in normal rat kidney cells transformed by a temperature-sensitive Rous sarcoma virus has been studied by polyacrylamide gel electrophoresis and autoradiography using cell monolayers prelabeled by lactoperoxidase-catalyzed radioiodination. Labeling of serum-starved cells under conditions that are nonpermissive for the expression of transformation reveals most of the radioactivity in the 250,000 molecular weight region. Parallel labeling of cells simultaneously exposed to serum limitation, under conditions that are permissive for the expression of transformation, reveals some radioactivity in the same slow-migrating region, but most of the label appears in the two faster migrating regions. The relative turnover of such external proteins has been investigated by examining the relative alterations in iodinated proteins after addition of normal levels of serum to a medium of serum-starved cells.

There is a greater relative turnover of the high-molecular-weight external component under conditions in which the transformation phenotype is expressed, as compared with conditions that limit the expression of transformation.

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Partial financial support by Consejo Nacional de Investigaciones Cientificas y Technologicas under Grant 31.26.S1-0469 is gratefully acknowledged.

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