Endomembrane organelles with an acidic lumen Visualization of the acidic luminal pH of endomembrane organelles The acidic luminal pH of the endomembrane organelles can be visualized using pH-sensitive fluorescent dyes, such as Acridine Orange and fluorescein isothiocyanate (FITC)-

bounded) organelles are involved in the dynamic membrane trafficking processes essential for protein sorting and targeting, receptor-mediated endocytosis, neural or hormonal signal transduction and other processes (Mellman et al., 1986; Goldstein et al., 1985). These organelles include the Golgi apparatus, secretory vesicles, coated vesicles, endosomes, lysosomes and synaptic vesicles. Their lumina are acidic, ranging in pH from 4.5 to 6.5, thus generating a transmembrane electrochemical proton gradient with respect to the near-neutral cytoplasm. The acidic luminal pH is established by a protonpumping vacuolar-type ATPase (V-ATPase) in combination with ion channels and transporters, whose varying distribution contributes to the organelle-specific luminal contents. The acidic pH is required for organelle functions such as hydrolysis of macromolecules, release of ligands from receptors and processing of preproproteins. The proton electrochemical gradient, consisting of the pH gradient (∆pH) and/or membrane potential (∆ψ), provides a driving force for the accumulation of hormones or transmitters into secretory vesicles, synaptic vesicles or synaptic-like microvesicles (microvesicles). A diversity of organelles is ensured by the accurate sorting and transport of the specific components into the lumina or membranes of the corresponding organelles. In this regard, it has become apparent recently that certain organelle-specific membrane protein(s) function in docking/fusion with transport vesicles (Ferro-Novick and Jahn, 1994). In this article, we briefly summarize recent studies on animal acidic organelles that possess V-ATPase. We include results from our own laboratories and emphasize our interests. Similarities between V-ATPase and F-ATPase (ATP synthase) are also discussed. Information about related fields not mentioned here can be found in reviews (Forgac, 1989; Anraku, 1996; Stevens and Forgac, 1997; Futai et al., 1998).