Incorporation of transmembrane peptides from the vacuolar H(+)-ATPase in phospholipid membranes: spin-label electron paramagnetic resonance and polarized infrared spectroscopy.

Peptides were designed that are based on candidate transmembrane sequences of the V o-sector from the vacuolar H (+)-ATPase of Saccharomyces cerevisiae. Spin-label EPR studies of lipid-protein interactions were used to characterize the state of oligomerization, and polarized IR spectroscopy was used to determine the secondary structure and orientation, of these peptides in lipid bilayer membranes. Peptides corresponding to the second and fourth transmembrane domains (TM2 and TM4) of proteolipid subunit c (Vma3p) and of the putative seventh transmembrane domain (TM7) of subunit a (Vph1p) are wholly, or predominantly, alpha-helical in membranes of dioleoyl phosphatidylcholine. All three peptides self-assemble into oligomers of different sizes, in which the helices are differently inclined with respect to the membrane normal. The coassembly of rotor (Vma3p TM4) and stator (Vph1p TM7) peptides, which respectively contain the glutamate and arginine residues essential to proton transport by the rotary ATPase mechanism, is demonstrated from changes in the lipid interaction stoichiometry and helix orientation. Concanamycin, a potent V-ATPase inhibitor, and a 5-(2-indolyl)-2,4-pentadienoyl inhibitor that exhibits selectivity for the osteoclast subtype, interact with the membrane-incorporated Vma3p TM4 peptide, as evidenced by changes in helix orientation; concanamycin additionally interacts with Vph1p TM7, suggesting that both stator and rotor elements contribute to the inhibitor site within the membrane. Comparison of the peptide behavior in lipid bilayers is made with membranous subunit c assemblies of the 16-kDa proteolipid from Nephrops norvegicus, which can substitute functionally for Vma3p in S. cerevisiae.

Interaction of spin-labeled inhibitors of the vacuolar H+-ATPase with the transmembrane Vo-sector.

The osteoclast variant of the vacuolar H(+)-ATPase (V-ATPase) is a potential therapeutic target for combating the excessive bone resorption that is involved in osteoporosis. The most potent in a series of synthetic inhibitors based on 5-(5,6-dichloro-2-indolyl)-2-methoxy-2,4-pentadienamide (INDOL0) has demonstrated specificity for the osteoclast enzyme, over other V-ATPases. Interaction of two nitroxide spin-labeled derivatives (INDOL6 and INDOL5) with the V-ATPase is studied here by using the transport-active 16-kDa proteolipid analog of subunit c from the hepatopancreas of Nephrops norvegicus, in conjunction with electron paramagnetic resonance (EPR) spectroscopy. Analogous experiments are also performed with vacuolar membranes from Saccharomyces cerevisiae, in which subunit c of the V-ATPase is replaced functionally by the Nephrops 16-kDa proteolipid. The INDOL5 derivative is designed to optimize detection of interaction with the V-ATPase by EPR. In membranous preparations of the Nephrops 16-kDa proteolipid, the EPR spectra of INDOL5 contain a motionally restricted component that arises from direct association of the indolyl inhibitor with the transmembrane domain of the proteolipid subunit c. A similar, but considerably smaller, motionally restricted population is detected in the EPR spectra of the INDOL6 derivative in vacuolar membranes, in addition to the larger population from INDOL6 in the fluid bilayer regions of the membrane. The potent classical V-ATPase inhibitor concanamycin A at high concentrations induces motional restriction of INDOL5, which masks the spectral effects of displacement at lower concentrations of concanamycin A. The INDOL6 derivative, which is closest to the parent INDOL0 inhibitor, displays limited subtype specificity for the osteoclast V-ATPase, with an IC(50) in the 10-nanomolar range.

An expanded and flexible form of the vacuolar ATPase membrane sector.

The vacuolar ATPase integral membrane c-ring from Nephrops norvegicus occurs in paired complexes in a double membrane. Using cryo-electron microscopy and single particle image processing of 2D crystals, we have obtained a projection structure of the c-ring of N. norvegicus. The c-ring was found to be very flexible, most likely as a result of an expanded conformation of the c subunits. This structure may support a role for the vacuolar ATPase c-rings in membrane fusion.

A divalent-ion binding site on the 16-kDa proton channel from Nephrops norvegicus--revealed by EPR spectroscopy.

As purified from the hepatopancreas of Nephrops norvegicus, the 16-kDa proton channel proteolipid is found to contain an endogenous divalent ion binding site that is occupied by Cu2+. The EPR spectrum has g-values and hyperfine splittings that are characteristic of type 2 Cu2+. The copper may be removed by extensive washing with EDTA. Titration with Ni2+ then induces spin-spin interactions with nitroxyl spin labels that are attached either to the unique Cys54, or to fatty acids intercalated in the membrane. Paramagnetic relaxation enhancement by the fast-relaxing Ni2+ is used to characterise the binding and to estimate distances from the dipolar interactions. The Ni2+-binding site on the protein is situated around 14-18 A from the spin label on Cys54, and is at a similar distance from a lipid chain spin-labelled on the 5 C-atom, but is more remote from the C-9 and C-14 positions of the lipid chains.

Concanamycin and indolyl pentadieneamide inhibitors of the vacuolar H+-ATPase bind with high affinity to the purified proteolipid subunit of the membrane domain.

The macrolide antibiotic concanamycin is a potent and specific inhibitor of the vacuolar H(+)-ATPase (V-ATPase), binding to the V(0) membrane domain of this eukaryotic acid pump. Although binding is known to involve the 16 kDa proteolipid subunit, contributions from other V(0) subunits are possible that could account for the apparently different inhibitor sensitivities of pump isoforms in vertebrate cells. In this study, we used a fluorescence quenching assay to directly examine the roles of V(0) subunits in inhibitor binding. Pyrene-labeled V(0) domains were affinity purified from Saccharomyces vacuolar membranes, and the 16 kDa proteolipid was subsequently extracted into chloroform and methanol and purified by size exclusion chromatography. Fluorescence from the isolated proteins was strongly quenched by nanomolar concentrations of both concanamycin and an indolyl pentadieneamide compound, indicating high-affinity binding of both natural macrolide and synthetic inhibitors. Competition studies showed that these inhibitors bind to overlapping sites on the proteolipid. Significantly, the 16 kDa proteolipid in isolation was able to bind inhibitors as strongly as V(0) did. In contrast, proteolipids carrying mutations that confer resistance to both inhibitors showed no binding. We conclude that the extracted 16 kDa proteolipid retains sufficient fold to form a high-affinity inhibitor binding site for both natural and synthetic V-ATPase inhibitors and that the proteolipid contains the major proportion of the structural determinants for inhibitor binding. The role of membrane domain subunit a in concanamycin binding and therefore in defining the inhibitor binding properties of tissue-specific V-ATPases is critically re-assessed in light of these data.

Assembly of the yeast vacuolar H+-ATPase and ATP hydrolysis occurs in the absence of subunit c''.

The V-ATPases are ubiquitous enzymes of eukaryotes. They are involved in many cellular processes via their ability to pump protons across biological membranes. They are two domain enzymes comprising an ATP hydrolysing sector and a proton translocating sector. Both sectors are functionally coupled. The proton tanslocating sector, V0, is comprised of five polypeptides in an as yet undetermined stoichiometry. In V0 three homologous proteins, subunit c, c' and c'' have previously been reported to be essential for assembly of the enzyme. However, we report that subunit c'' is not essential for assembly but is for functional coupling of the enzyme.

Interaction of inhibitors of the vacuolar H(+)-ATPase with the transmembrane Vo-sector.

The macrolide antibiotic concanamycin A and a designed derivative of 5-(2-indolyl)-2,4-pentadienamide (INDOL0) are potent inhibitors of vacuolar H(+)-ATPases, with IC(50) values in the low and medium nanomolar range, respectively. Interaction of these V-ATPase inhibitors with spin-labeled subunit c in the transmembrane V(o)-sector of the ATPase was studied by using the transport-active 16-kDa proteolipid analogue of subunit c from the hepatopancreas of Nephrops norvegicus. Analogous experiments were also performed with vacuolar membranes from Saccharomyces cerevisiae. Membranous preparations of the Nephrops 16-kDa proteolipid were spin-labeled either on the unique cysteine C54, with a nitroxyl maleimide, or on the functionally essential glutamate E140, with a nitroxyl analogue of dicyclohexylcarbodiimide (DCCD). These residues were previously demonstrated to be accessible to lipid. Interaction of the inhibitors with these lipid-exposed residues was studied by using both conventional and saturation transfer EPR spectroscopy. Immobilization of the spin-labeled residues by the inhibitors was observed on both the nanosecond and microsecond time scales. The perturbation by INDOL0 was mostly greater than that by concanamycin A. Qualitatively similar but quantitatively greater effects were obtained with the same spin-label reagents and vacuolar membranes in which the Nephrops 16-kDa proteolipid was expressed in place of the native vma3p proteolipid of yeast. The spin-label immobilization corresponds to a direct interaction of the inhibitors with these intramembranous sites on the protein. A mutational analysis on transmembrane segment 4 known to give resistance to concanamycin A also gave partial resistance to INDOL0. The results are consistent with transmembrane segments 2 and 4 of the 16-kDa putative four-helix bundle, and particularly the functionally essential protonation locus, being involved in the inhibitor binding sites. Inhibition of proton transport may also involve immobilization of the overall rotation of the proteolipid subunit assembly.

The effects of a mutant connexin 26 on epidermal differentiation.

To elucidate the mode of action of dominant mutant connexins in causing inherited skin diseases, transgenic mice were produced that express the true Vohwinkel syndrome-associated mutant Cx26 (D66H), from a keratin 10 promoter, specifically in the suprabasal epidermal keratinocytes. Following birth, the transgenic mice developed keratoderma similar to that of human carriers of Cx26 (D66H). Expression of the transgene resulted in a loss of Cx26 and Cx30 at intercellular junctions of epidermal keratinocytes and accumulation of these connexins in the cytoplasm. Injection of primary mouse keratinocytes with Lucifer Yellow showed no difference in terms of dye spreading between transgenic and non transgenic keratinocytes in vitro. Expression of the mutant Cx26 (D66H) did not interfere with the formation of the epidermal water barrier during late embryonic development. Attempts to produce transgenic mice expressing the wild type form of Cx26 from the K10 promoter failed to produce viable animals although transgenic embryos were recovered at days 9 and 12 of gestation, suggesting that the transgene might be embryonic lethal.

Targeted epidermal expression of mutant Connexin 26(D66H) mimics true Vohwinkel syndrome and provides a model for the pathogenesis of dominant connexin disorders.

To investigate the role of connexins in dominantly inherited skin disease, transgenic mice were produced which expressed mutant connexin 26 [gjb2/connexin 26(D66H)], from a keratin 10 promoter, exclusively in the suprabasal epidermis (the cells in which Connexin 26 is up-regulated in epidermal hyperproliferative states). From soon after birth, the mice exhibited a keratoderma similar to that in humans carrying the Connexin 26(D66H) mutation (true Vohwinkel syndrome). Transgene expression was associated with loss of Connexin 26 and Connexin 30 from epidermal keratinocyte intercellular junctions and accumulation in cytoplasm. Light and electron microscopy showed marked thickening of the epidermal cornified layers and increased epidermal TUNEL staining, indicative of premature keratinocyte programmed cell death. The K10Connexin 26(D66H) mouse may provide a valuable model to study the role of gap-junctional intercellular communication in epidermal differentiation. Similarities in phenotype between individuals (man and mouse) carrying Connexin 26(D66H) and those carrying insertional mutants of Loricrin, a major cornified envelope protein of the epidermis, suggest a possible link between connexin function and cornified envelope formation.

Evidence that there are two copies of subunit c" in V0 complexes in the vacuolar H+-ATPase.

The proton-translocating core of eukaryotic vacuolar H(+)-ATPase (V-ATPase), V(0) consists of a hexameric arrangement of transmembrane alpha-helices formed from the related polypeptides, subunit c and subunit c". The former is comprised of four transmembrane alpha-helices, whilst the latter has an extra transmembrane domain at its N-terminus. In addition, the fungal form of V(0) contains a minor subunit c-related polypeptide, subunit c'. All three are required for activity of the proton pump in Saccharomyces cerevisiae. We have introduced cysteine residues in the N-terminal extension of subunit c" in a cysteine-free form. All mutant forms are active in the V-ATPase from S. cerevisiae. Oxidation of vacuolar membranes containing the cysteine-replaced forms gave a cross-linked product of 42000Da. Analysis of this species showed it to be a dimeric form of subunit c", and further studies confirmed there are two copies of subunit c" in the V-ATPases in which it is present. Co-expression of double cysteine-replaced forms of both subunit c and c" gave rise to only homotypic cross-linked forms. Also, subunit c oligomeric complexes are present in vacuolar membranes in the absence of subunit c", consistent with previous observations showing hexameric arrangements of subunit c in gap-junction-like membranes. In vitro studies showed subunit c" can bind to subunit c and itself. The extent of binding can be increased by removal of the N-terminal domain of subunit c". This domain may therefore function to limit the copy number of subunit c" in V(0). A deletion study shows that the domain is essential for the activity of subunit c". The results can be combined into a model of V(0) which contains two subunit c" protomers with the extra transmembrane domain located toward the central pore. Thus the predicted stoichiometry of V(0) in which subunit c" is present is subunit c(3):subunit c'(1):subunit c"(2). On the basis of the mutational and binding studies, it seems likely that two copies of subunit c" are next to each other.

E5 transforming proteins of papillomaviruses do not disturb the activity of the vacuolar H(+)-ATPase.

Papillomaviruses contain a gene, E5, that encodes a short hydrophobic polypeptide that has transforming activity. E5 proteins bind to the 16 kDa subunit c (proteolipid) of the eukaryotic vacuolar H(+)-ATPase (V-ATPase) and this binding is thought to disturb the V-ATPase and to be part of transformation. This link has been examined in the yeast Saccharomyces cerevisiae. The E5 proteins from human papillomavirus (HPV) type 16, bovine papillomavirus (BPV) type 1, BPV-4 E5 and various mutants of E5 and the p12' polypeptide from human T-lymphotropic virus (HTLV) type I all bound to the S. cerevisiae subunit c (Vma3p) and could be found in vacuolar membranes. However, none affected the activity of the V-ATPase. In contrast, a dominant-negative mutant of Vma3p (E137G) inactivated the enzyme and gave the characteristic VMA phenotype. A hybrid V-ATPase containing a subunit c from Norway lobster also showed no disruption. Sedimentation showed that HPV-16 E5 was not part of the active V-ATPase. It is concluded that the binding of E5 and E5-related proteins to subunit c does not affect V-ATPase activity or function and it is proposed that the binding may be due to a chaperone function of subunit c.