A view of acidic intracellular compartments.

Abstract

C ONSIDERABLE evidence has accumulated over the past ten years that the interior of both the endocytic and exocytic vacuolar apparatus in cells is maintained at a low pH (30, 47, 60). The interior of endocytic vesicles (4, 17, 29, 57, 59), lysosomes (37, 43), portions of the transGolgi apparatus (3, 38, 39), certain secretory vesicles (14, 22, 23, 38, 48), and plant tonoplasts (7) is acidic. The maintenance of the high H § concentration depends upon the properties of the surrounding membrane and the vectorial movement of protons mediated by an ATP dependent H § pump (1, 30, 47). A variety of different techniques have been used to study the function of low pH compartments. (a) Using purified secretory vesicles (24, 36), yeast vacuoles (49), and lysosomes (42, 53), the influx and effiux of low molecular weight solutes has been shown to depend on both a transmembrane H § gradient and an electrical gradient generated by an ATPdependent H § pump in the membrane of these vesicles. (b) Several laboratories have isolated mutant cell lines that are defective in H § pumping (28, 31, 45); these cells are resistant to certain toxins that gain entry to cells by acidic endosomes and they are unable to obtain iron from transferrin during receptor-mediated endocytosis. (c) Reagents that either neutralize low pH compartments or dissipate H § gradients inhibit a variety of endocytic and exocytic activities including receptor recycling during receptor-mediated endocytosis (9), the sorting of both content and membrane proteins during exocytosis (19, 33, 58) and endocytosis (9), and the posttranslational processing of macromolecules that pass through the Golgi apparatus (39). To better understand the function of acidic compartments, investigators have studied H § gradients in the living cell using vital pH indicators. When taken up by cells, these indicators either selectively accumulate in low pH compartments or detect the pH of the compartment in which they reside. As outlined below, recent advances in immunocytochemistry, flow cytometry, and videoenhanced microscopy offer the interested investigator new opportunities for using these indicators to "capture" the H § gradient as it exists in the living cell and detect the chemical reactions that occur in that environment.