The membrane proteins of the vacuolar system. II. Bidirectional flow between secondary lysosomes and plasma membrane

Lactoperoxidase covalently coupled to latex spheres (LPO-latex) has been used to selectively iodinate the phagolysome (PL) membrane within living macrophages, as discussed in the accompanying article. This procedure labeled approximately 24 polypeptides in the PL membrane; these were similar to those iodinatable on the external surface of the plasma membrane (PM). We now report on the translocation and fate of these proteins when the cells are returned to culture. TCA-precipitable radioactivity was lost from cells with biphasic kinetics. 20-50% of the cell-associated radiolabel was rapidly digested (t 1/2 approximately equal to 1 h) and recovered in the culture medium as monoiodotyrosine. 50-80% of the label was lost slowly from cells ( 1/2 approximately equal to 24-30 h). Quantitative analysis of gel autoradiograms showed that all radiolabeled proteins were lost at the same rate in both the rapid and slow phases of digestion. Within 15-30 min aftr labeling of the PL membrane, EM autoradiography revealed that the majority of the cell-associated grains, which at time 0 were associated with PL, were now randomly dispersed over the plasmalemma. At this time, analysis of PM captured by a second phagocytic load revealed the presence of all labeled species originally present in the PL membrane. This demonstrated the rapid, synchronous centrifugal flow of PL polypeptides to the cell surface. Evidence was also obtained for the continuous influx of representative samples of the PM into the PL compartment by way of pinocytic vesicles. This was based on the constant flow of fluid phase markers into latex-containing PL and on the internalization of all iodinatable PM polypeptides into this locus. These observations provide evidence for the continuous, bidirectional flow of membrane polypeptides between the PM and the secondary lysosome and represent an example of a membrane flow and recycling mechanism.