Acidification of three types of liver endocytic vesicles: similarities and differences

Abstract

Endocytosed ligands move through a series of progressively more acidic vesicles. These differences in pH (pHi) could reflect differences in ion transport mechanisms. Vesicles representing three stages of endocytosis, compartment for uncoupling of receptor and ligand (CURL), multivesicular bodies (MVB), and receptor recycling compartment (RRC), were studied, and all exhibited ATP-dependent electrogenic acidification that was a saturable function of medium chloride. Initial rates of acidification differed (RRC > CURL > MVB), and proton influx was similar for CURL and RRC but slower for MVB. Steady-state ATP-dependent pHi in the three vesicles was more similar. Vesicle membrane potential was substantial (+41 to +69 mV) in low-chloride medium and greatest for RRC but was low (-6 to +6 mV) in 140 mM KCl. These vesicles also exhibited -22 to -37 mV Donnan potentials. Steady-state pump-generated proton electrochemical gradients (delta mu H+) ranged from 114 to 175 mV and were greater for CURL and RRC than for MVB; however, delta mu H+ changed little over a 140-fold difference in chloride concentration. Proton leak rates were faster in CURL and RRC than in MVB, but proton efflux was similar. Finally, proton fluxes and permeabilities, calculated with regard to surface area, differed in the opposite direction (MVB > CURL > RRC). Thus, for the endocytic vesicles studied, intrinsic differences in proton flux and in vesicle geometry could be demonstrated that contributed to differences in pre-steady-state vesicle pHi.

Evidence that translocation of anthrax toxin's lethal factor is initiated by entry of its N terminus into the protective antigen channel

“…There is consensus that the interior of the compartment is positive with respect to the cytosol: the same polarity that promotes channel blocking. Estimates of the magnitude of the potential vary widely, however, from ϩ10 to ϩ300 mV (23)(24)(25). Whereas the pore might discharge the transmembrane potential, the N terminus of LF or EF inserted into the prepore might block the pore before it opens.…”

V-ATPase expression during development ofArtemia franciscanaembryos: potential role for proton gradients in anoxia signaling

“…However, in any given cell, the effect of proton flux from these compartments would ultimately depend on their relative buffering capacities and size in relation to the surrounding cytoplasmic space. While we are unaware of any studies reporting these values in Artemia embryos, a large body of literature does describe such values for components of the protein synthetic, degradative, exocytotic and endocytotic pathways of other species (Ibarrola et al, 2000;Kelly et al, 1991;Rybak et al, 1997;Schoonderwoert and Martens, 2001;Van Dyke and Belcher, 1994;Wu et al, 2001). Based both on these data and published morphological studies on Artemia (Perona et al, 1988;Perona and Vallejo, 1989), one can estimate that Golgi, lysosomes and various transport vesicles constitute 2% of the intracellular volume in Artemia embryos and have a weighted average pH of 5.4 with a buffering capacity of 60·mmol·H over the predicted range of compartment alkalinization under anoxia.…”

Stochastic Model of Acidification, Activation of Hemagglutinin and Escape of Influenza Viruses from an Endosome

“…where β 0 e P e (t) = n i = 1 β i P e (t) is the total buffering capacity of the endosome, which is approximately constant β 0 e P e (t) = β 0 e = 40mM/pH [40]. Finally, using the proton extrusion and pumping rates (Equations 33 and 34), we obtain the kinetic Equation…”

“…Due to possible ionic exchange between viral and endosomal lumens, we approximate β 0 v (pH) with the endosomal buffering capacity β 0 e , which is independent of the pH and has been estimated to be [40] β 0 e = 40 mM/pH.…”