Interaction of the Thermoplasma acidophilum A1A0-ATP synthase peripheral stalk with the catalytic domain

Probing subunit-subunit interactions in the yeast vacuolar ATPase by peptide arrays

BACKGROUND

Vacuolar (H(+))-ATPase (V-ATPase; V(1)V(o)-ATPase) is a large multisubunit enzyme complex found in the endomembrane system of all eukaryotic cells where its proton pumping action serves to acidify subcellular organelles. In the plasma membrane of certain specialized tissues, V-ATPase functions to pump protons from the cytoplasm into the extracellular space. The activity of the V-ATPase is regulated by a reversible dissociation mechanism that involves breaking and re-forming of protein-protein interactions in the V(1)-ATPase - V(o)-proton channel interface. The mechanism responsible for regulated V-ATPase dissociation is poorly understood, largely due to a lack of detailed knowledge of the molecular interactions that are responsible for the structural and functional link between the soluble ATPase and membrane bound proton channel domains.

METHODOLOGY/PRINCIPAL FINDINGS

To gain insight into where some of the stator subunits of the V-ATPase associate with each other, we have developed peptide arrays from the primary sequences of V-ATPase subunits. By probing the peptide arrays with individually expressed V-ATPase subunits, we have identified several key interactions involving stator subunits E, G, C, H and the N-terminal domain of the membrane bound a subunit.

CONCLUSIONS

The subunit-peptide interactions identified from the peptide arrays complement low resolution structural models of the eukaryotic vacuolar ATPase obtained from transmission electron microscopy. The subunit-subunit interaction data are discussed in context of our current model of reversible enzyme dissociation.

Crystal structure of the yeast vacuolar ATPase heterotrimeric EGC(head) peripheral stalk complex

Vacuolar ATPases (V-ATPases) are multisubunit rotary motor proton pumps that function to acidify subcellular organelles in all eukaryotic organisms. V-ATPase is regulated by a unique mechanism that involves reversible dissociation into V₁-ATPase and V₀ proton channel, a process that involves breaking of protein interactions mediated by subunit C, the cytoplasmic domain of subunit "a" and three "peripheral stalks," each made of a heterodimer of E and G subunits. Here, we present crystal structures of a yeast V-ATPase heterotrimeric complex composed of EG heterodimer and the head domain of subunit C (C(head)). The structures show EG heterodimer folded in a noncanonical coiled coil that is stabilized at its N-terminal ends by binding to C(head). The coiled coil is disrupted by a bulge of partially unfolded secondary structure in subunit G and we speculate that this unique feature in the eukaryotic V-ATPase peripheral stalk may play an important role in enzyme structure and regulation by reversible dissociation.

Binding of subunit E into the A-B interface of the A(1)A(O) ATP synthase

Two of the distinct diversities of the engines A(1)A(O) ATP synthase and F(1)F(O) ATP synthase are the existence of two peripheral stalks and the 24kDa stalk subunit E inside the A(1)A(O) ATP synthase. Crystallographic structures of subunit E have been determined recently, but the epitope(s) and the strength to which this subunit does bind in the enzyme complex are still a puzzle. Using the recombinant A(3)B(3)D complex and the major subunits A and B of the methanogenic A(1)A(O) ATP synthase in combination with fluorescence correlation spectroscopy (FCS) we demonstrate, that the stalk subunit E does bind to the catalytic headpiece formed by the A(3)B(3) hexamer with an affinity (K(d)) of 6.1±0.2μM. FCS experiments with single A and B, respectively, demonstrated unequivocally that subunit E binds stronger to subunit B (K(d)=18.9±3.7μM) than to the catalytic A subunit (K(d)=53.1±4.4). Based on the crystallographic structures of the three subunits A, B and E available, the arrangement of the peripheral stalk subunit E in the A-B interface has been modeled, shining light into the A-B-E assembly of this enzyme.

Structural divergence of the rotary ATPases

The rotary ATPase family of membrane protein complexes may have only three members, but each one plays a fundamental role in biological energy conversion. The F1Fo-ATPase (F-ATPase) couples ATP synthesis to the electrochemical membrane potential in bacteria, mitochondria and chloroplasts, while the vacuolar H+-ATPase (V-ATPase) operates as an ATP-driven proton pump in eukaryotic membranes. In different species of archaea and bacteria, the A1Ao-ATPase (A-ATPase) can function as either an ATP synthase or an ion pump. All three of these multi-subunit complexes are rotary molecular motors, sharing a fundamentally similar mechanism in which rotational movement drives the energy conversion process. By analogy to macroscopic systems, individual subunits can be assigned to rotor, axle or stator functions. Recently, three-dimensional reconstructions from electron microscopy and single particle image processing have led to a significant step forward in understanding of the overall architecture of all three forms of these complexes and have allowed the organisation of subunits within the rotor and stator parts of the motors to be more clearly mapped out. This review describes the emerging consensus regarding the organisation of the rotor and stator components of V-, A- and F-ATPases, examining core similarities that point to a common evolutionary origin, and highlighting key differences. In particular, it discusses how newly revealed variation in the complexity of the inter-domain connections may impact on the mechanics and regulation of these molecular machines.

Disulfide linkage in the coiled-coil domain of subunit H of A1AO ATP synthase from Methanocaldococcus jannaschii and the NMR structure of the C-terminal segment H(85-104)

The C-terminal residues 98-104 are important for structure stability of subunit H of A(1)A(O) ATP synthases as well as its interaction with subunit A. Here we determined the structure of the segment H(85-104) of H from Methanocaldococcus jannaschii, showing a helix between residues Lys90 to Glu100 and flexible tails at both ends. The helix-helix arrangement in the C-terminus was investigated by exchange of hydrophobic residues to single cysteine in mutants of the entire subunit H (H(I93C), H(L96C) and H(L98C)). Together with the surface charge distribution of H(85-104), these results shine light into the A-H assembly of this enzyme.