Interaction of dibutyltin-3-hydroxyflavone bromide with the 16 kDa proteolipid indicates the disposition of proton translocation sites of the vacuolar ATPase

Pseudoenzymatic dealkylation of alkyltins by biological dithiols

We investigated the time dependence of the degradation of three alkyltin derivatives by a nine amino acid linear peptide (I(1)LGCWCYLR(9)) containing a CXC motif derived from the primary sequence of stannin, a membrane protein involved in alkyltin toxicity. We monitored the reaction kinetics using the intrinsic fluorescence of the tryptophan residue in position 5 of the peptide and found that all of the alkyltins analyzed are progressively degraded to dialkyl derivatives, following a pseudoenzymatic reaction mechanism. The end point of the reactions is the formation of a covalent complex between the disubstituted alkyltin and the peptide cysteines. These data agree with the speciation profiles proposed for polysubstituted alkyltins in the environment and reveal a possible biotic degradation pathway for these toxic compounds.

Mechanism of inhibition of the V-type molecular motor by tributyltin chloride

Tributyltin chloride (TBT-Cl) is an endocrine disruptor found in many animal species, and it is also known to be an inhibitor for the V-ATPases that are emerging as potential targets in the treatment of diseases such as osteoporosis and cancer. We demonstrated by using biochemical and single-molecular imaging techniques that TBT-Cl arrests an elementary step for rotary catalysis of the V(1) motor domain. In the presence of TBT-Cl, the consecutive rotation of V(1) paused for a long duration ( approximately 0.5 s), even at saturated ATP concentrations, and the pausing positions were localized at 120 degrees intervals. Analysis of both the pausing time and moving time revealed that TBT-Cl has little effect on the binding affinity for ATP, but, rather, it arrests the catalytic event(s). This is the first report to demonstrate that an inhibitor arrests an elementary step for rotary catalysis of a V-type ATP-driven rotary motor.

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.

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.

Identification of lipid-accessible sites on the nephrops 16-kDa proteolipid incorporated into a hybrid vacuolar H(+)-ATPase: site-directed labeling with N-(1-Pyrenyl)cyclohexylcarbodiimide and fluorescence quenching analysis

Proton translocation by the vacuolar H(+)-ATPase is mediated by a multicopy transmembrane protein, the 16-kDa proteolipid. It is proposed to assemble in the membrane as a hexameric complex, with each polypeptide comprising four transmembrane helices. The fourth helix of the proteolipid contains an intramembrane acidic residue (Glu140) which is essential for proton translocation and is reactive toward N,N'-dicyclohexylcarbodiimide (DCCD). Current theoretical models of proton translocation by the vacuolar ATPase require that Glu140 should be protonated and in contact with the membrane lipid. In this study we present direct support for this hypothesis. Modification with the fluorescent DCCD analogue N-(1-pyrenyl)cyclohexylcarbodiimide, coupled to fluorescence quenching studies and bilayer depth measurements using the parallax method, was used to probe the position of Glu140 with respect to the bilayer. Glutamate residues were also introduced mutagenically as targets for the fluorescent probe in order to map additional lipid-accessible sites on the 16-kDa proteolipid. These data are consistent with a structural model of the 16-kDa proteolipid oligomer in which the key functional residue Glu140 and discrete faces of the second and third transmembrane helices of the 16-kDa proteolipid are exposed at the lipid-protein interface.

Membrane assembly of the 16-kDa proteolipid channel from Nephrops norvegicus studied by relaxation enhancements in spin-label ESR

The 16-kDa proteolipid from the hepatopancreas of Nephrops norvegicus belongs to the class of channel proteins that includes the proton-translocation subunit of the vacuolar ATPases. The membranous 16-kDa protein from Nephrops was covalently spin-labeled on the unique cysteine Cys54, with a nitroxyl maleimide, or on the functionally essential glutamate Glu140, with a nitroxyl analogue of dicyclohexylcarbodiimide (DCCD). The intensities of the saturation transfer ESR spectra are a sensitive indicator of spin-spin interactions that were used to probe the intramembranous structure and assembly of the spin-labeled 16-kDa protein. Spin-lattice relaxation enhancements by aqueous Ni(2+) ions revealed that the spin label on Glu140 is located deeper within the membrane (around C9-C10 of the lipid chains) than is that on Cys54 (located around C5-C6). In double labeling experiments, alleviation of saturation by spin-spin interactions with spin-labeled lipids indicates that spin labels both on Cys54 and on Glu140 are at least partially exposed to the lipid chains. The decrease in saturation transfer ESR intensity observed with increasing spin-labeling level is evidence of oligomeric assembly of the 16-kDa monomers and is consistent with a protein hexamer. These results determine the locations and orientations of transmembrane segments 2 and 4 of the 16-kDa putative 4-helix bundle and put constraints on molecular models for the hexameric assembly in the membrane. In particular, the crucial DCCD-binding site that is essential for proton translocation appears to contact lipid.

Selective inhibitors of the osteoclast vacuolar proton ATPase as novel bone antiresorptive agents

The proton ATPase located on the apical membrane of the osteoclast is essential to the bone resorption process. This proton pump is, therefore, an attractive molecular target for the design of novel inhibitors of bone resorption, and potentially useful for the treatment of osteoporosis and related metabolic diseases of bone. Recently, several inhibitors with different degrees of selectivity for the osteoclast V-ATPase have been reported. In particular, systematic chemical modifications of the macrolide antibiotic bafilomycin A1 have identified the minimal structural requirements for activity and allowed the design of simplified analogues that demonstrate high potency and selectivity for the osteoclast enzyme.

The vacuolar H+-ATPase: a universal proton pump of eukaryotes

The vacuolar H+-ATPase (V-ATPase) is a universal component of eukaryotic organisms. It is present in the membranes of many organelles, where its proton-pumping action creates the low intra-vacuolar pH found, for example, in lysosomes. In addition, there are a number of differentiated cell types that have V-ATPases on their surface that contribute to the physiological functions of these cells. The V-ATPase is a multi-subunit enzyme composed of a membrane sector and a cytosolic catalytic sector. It is related to the familiar FoF1 ATP synthase (F-ATPase), having the same basic architectural construction, and many of the subunits from the two display identity with one another. All the core subunits of the V-ATPase have now been identified and much is known about the assembly, regulation and pharmacology of the enzyme. Recent genetic analysis has shown the V-ATPase to be a vital component of higher eukaryotes. At least one of the subunits, i.e. subunit c (ductin), may have multifunctional roles in membrane transport, providing a possible pathway of communication between cells. The structure of the membrane sector is known in some detail, and it is possible to begin to suggest how proton pumping is coupled to ATP hydrolysis.