In previous publications (Muller, W.A., R.M. Steinman, Z.A. Cohn. 1980, J.Cell Biol. 86:292-314), we found that the membrane of macrophage phagolysosomes could be selectively radioiodinated in living cells, The technique required phagocytosis of lactoperoxidase covalently coupled to latex spheres (LPO-latex), followed by iodination on ice with Na(125)I and hydrogen peroxide. In this paper, we use the LPO-latex system to further analyze the composition and recycling of phagocytic vacuole membrane.
Three approaches were employed to examine the polypeptide composition of the phagolysosome (PL) and plasma membranes (PM). (a) The efficiency of intracellular iodination was increased by increasing lysosomal pH with chloroquine. By one-dimensional SDS PAGE, the heavily labeled chloroquine-treated PL exhibited the same labeled polypeptides as PM iodinated extracellularly with LPO-latex. (b) Iodinated PL and PM were compared by two-dimensional gel electrophoresis. No differences in the isoelectric point and molecular weight of the major iodinated species were detected. (c) Quantitative immune precipitation was performed with five specific antibodies directed against cell surface antigens. Four antibodies precipitated similar relative amounts of labeled antigen on the cell surface and endocytic vacuole. One antibody, secreted by hybridoma 2.6, detected a 21-kdalton polypeptide that was enriched sevenfold in PL membrane. This enrichment was cell surface-derived, since the amount of labeled 2.6 was increased sevenfold when iodinated PM was driven into the cell during latex uptake. Therefore, intracellular iodination primarily detects PL proteins that are identical to their PM counterparts.
Additional studies employed electron microscope autoradiography to monitor the centrifugal flow of radiolabeled polypeptides from PL to PM. Cells were iodinated intralysosomally and returned to culture for only 5-10 min at 37 degrees C. Most of the cell-associated label then redistributed to the cell surface or its adjacent area. Significant movement out of the lysosome compartment occurred even at 2 degrees C and 22 degrees C. Extensive and rapid membrane flow through the secondary lysosome presumably contributes to the great similarity between PM and PL membrane polypeptides.