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  • br Introduction The vacuolar ATPases V ATPases are ATP drive

    2023-12-08


    Introduction The vacuolar ATPases (V-ATPases) are ATP-driven proton pumps that play important roles in both normal and disease processes [1], [2], [3], [4], [5]. V-ATPases within cells function in such processes as intracellular membrane traffic, protein processing and degradation, coupled transport of small molecules and the entry of various viruses and toxins, including influenza virus and diphtheria toxin [1], [2], [3], [6]. V-ATPases present in the plasma membrane of specialized cells are important for D-NMAPPD secretion in the kidney, sperm maturation in the epididymis and degradation of bone by osteoclasts [1], [2], [4], [5]. The V-ATPases are large, multi-subunit complexes composed of a peripheral V1 domain that hydrolyzes ATP and an integral V0 domain that translocates protons [1]. The V1 domain is composed of eight subunits (A–H) in a stoichiometry of A3B3C1D1E3F1G3H1 while the V0 domain (in mammals) contains five subunits in a stoichiometry of a1c9c”1d1e1 (see Fig. 1, adapted from reference [7]). The V-ATPases operate by a rotary mechanism in which ATP hydrolysis at catalytic sites located at the interface of the A and B subunits drives rotation of a central rotor composed on subunits D and F of V1 connected to the ring of proteolipid subunits (c,c”) in V0[1], [2]. Each proteolipid subunit contains a single buried glutamate residue which undergoes reversible protonation during proton transport and is the site of modification by the inhibitor dicyclohexylcarbodiimide (DCCD) [8]. Protons reach these buried residues by way of a proton-conducting hemi-channel located in the C-terminal hydrophobic domain of subunit a [9]. Following ATP-driven rotation of the proteolipid ring, the buried glutamate residues are deprotonated through interaction with a single buried arginine residue located in subunit a [10] and then exit via a second hemi-channel in subunit a to the luminal side. Subunit a is held fixed relative to the A3B3 catalytic head through interactions between the N-terminal domain of subunit a and subunits C, H and the EG heterodimers [11], [12]. An important mechanism of regulating V-ATPase activity in vivo involves reversible dissociation and reassembly of the V1 and V0 domains (Fig. 2). The first part of this article will focus on recent advances from our laboratory on understanding the regulation of V-ATPase assembly in yeast in response to changes in glucose concentration [13], during maturation of dendritic cells [14] and in mammalian cells in response to changes in amino acid levels [15]. Targeting of V-ATPases to different cellular membranes is controlled by isoforms of subunit a [1], [4], [5]. In mammals, subunit a exists as four isoforms (a1–a4), with a3 and a4 responsible for targeting of V-ATPases to the plasma membrane of osteolcasts and renal intercalated cells, respectively [4], [16]. Results from a variety of laboratories have suggested a role of V-ATPases in cancer cell survival and invasion [17], [18], [19], [20]; for reviews see [1], [21], [22]. The second part of this article will focus on recent advances from our laboratory on the role of plasma membrane and a3-containing V-ATPases in breast cancer cell migration and invasion [17], [18], [19].
    Regulation of V-ATPase assembly in yeast Regulated assembly of the V-ATPase represents an important and widely employed mechanism of controlling V-ATPase activity in vivo (Fig. 2). In response to a variety of stimuli, the V-ATPase undergoes reversible changes in the degree of assembly of the V1 and V0 domains, which, in the dissociated state, remain inactive with respect to ATP hydrolysis and proton translocation [1]. In yeast, dissociation of the V-ATPase complex occurs in response to glucose depletion, is rapid and reversible, and does not require new protein synthesis [23]. Reassembly requires interaction with the RAVE complex, which interacts with the V-ATPase in a glucose-dependent manner and whose interactions have now been carefully mapped [24]. Interestingly RAVE promotes the assembly of only V-ATPase complexes containing the yeast isoform of subunit a which targets the complex to the vacuole (Vph1p), and not those containing Stv1p, which are targeted to the Golgi [25]. Similarly, glucose-dependent assembly also requires interaction of the V-ATPase with the glycolytic enzyme aldolase [26]. Interactions with PI(3,5)P2 appear to stabilize assembly of the V-ATPase on the vacuolar membrane through interaction with the N-terminal domain of subunit a [27].