Rabbit alveolar macrophages were incubated in vitro with radioactive protein precursors. Plasma membranes were isolated from these cells, dissolved in phenol-urea-acetic acid, and separated by acrylamide gel electrophoresis. 3H-leucine was rapidly incorporated into membrane protein. The rate of labeling with 3H-leucine was markedly different from one protein band to another, indicating heterogeneous or multistep synthesis and assembly of proteins in the alveolar macrophage plasma membrane.
Cells incubated with 3H-choline incorporated this compound into membrane lecithin. On gel electrophoresis the label derived from choline was located in the two bands migrating most rapidly towards the cathode. Studies on cells incubated with 3H-glucosamine revealed incorporation of label into two protein bands, one located near the origin and the other migrating rapidly towards the cathode.
The in vitro techniques were also employed for pulse-chase studies to gain information on rate of turnover of macrophage plasma membrane proteins. This turnover rate was rapid, with a half-life of approximately 8 hr. The radioactivity disappeared from the several protein bands at the same rate, suggesting bulk removal of membrane rather than catabolism of the individual proteins in situ. Endocytosis seems a likely mechanism to account for a major part of the plasma membrane removal.
Studies on the protein components of phagolysosomal membranes from cells which had been labeled with 3H-leucine revealed the presence of all of the major labeled protein bands characteristic of the plasma membrane except one, thus confirming the bulk interiorization of large segments or units of plasma membrane by endocytic processes.