Studies of membrane formation in Tetrahymena. The identification of membrane proteins and turnover rates in nongrowing cells.

Membrane proteins within cell fractions of Tetrahymena have been separated by polyacrylamide gel electrophoresis and the turnover of these proteins has been studied in nongrowing cells using double labeling procedures. Lactoperoxidase iodination and two-dimensional gel separations have revealed over 20 surface-labeled proteins, two of which are responsible for the single peak found in ciliary and nonciliary surface membranes. Turnover measurements show that there is heterogeneity in the rates at which membrane proteins turn over. The data suggest that larger proteins may turn over at higher rates in the microsomal fraction, as previously shown in mammalian cells. This was not found in the surface membrane fractions of Tetrahymena. However, membrane-associated cytoskeletal proteins had lower turnover rates than integral membrane proteins, and significant heterogeneity was found among the integral proteins measured. The implications of these findings for possible modes of membrane biogenesis in Tetrahymena are discussed.