Formation in vivo, purification and crystallization of a complex of the gamma and epsilon subunits of the F0F1-ATPase of Escherichia coli

Study of the yeast Saccharomyces cerevisiae F1F0-ATPase epsilon-subunit

The yeast Saccharomyces cerevisiae F1F0-ATPase epsilon-subunit (61 residues) was synthesized by the solid-phase peptide approach under both acidic and basic strategies. Only the latter strategy allowed us to obtain a pure epsilon-subunit. The strong propensity of the protein to produce few soluble dimeric species depending on pH has been proved by size-exclusion chromatography, electrophoresis and mass spectrometry. A circular dichroism study showed that an aqueous solution containing 30% trifluoroethanol or 200 mM sodium dodecyl sulphate is required for helical folding. In both solvents at acidic pH, the epsilon-subunit is soluble and monomeric.

Molecular architecture of the rotary motor in ATP synthase

Adenosine triphosphate (ATP) synthase contains a rotary motor involved in biological energy conversion. Its membrane-embedded F0 sector has a rotation generator fueled by the proton-motive force, which provides the energy required for the synthesis of ATP by the F1 domain. An electron density map obtained from crystals of a subcomplex of yeast mitochondrial ATP synthase shows a ring of 10 c subunits. Each c subunit forms an alpha-helical hairpin. The interhelical loops of six to seven of the c subunits are in close contact with the gamma and delta subunits of the central stalk. The extensive contact between the c ring and the stalk suggests that they may rotate as an ensemble during catalysis.

Effect of the epsilon-subunit on nucleotide binding to Escherichia coli F1-ATPase catalytic sites

The influence of the epsilon-subunit on the nucleotide binding affinities of the three catalytic sites of Escherichia coli F1-ATPase was investigated, using a genetically engineered Trp probe in the adenine-binding subdomain (beta-Trp-331). The interaction between epsilon and F1 was not affected by the mutation. Kd for binding of epsilon to betaY331W mutant F1 was approximately 1 nM, and epsilon inhibited ATPase activity by 90%. The only nucleotide binding affinities that showed significant differences in the epsilon-depleted and epsilon-replete forms of the enzyme were those for MgATP and MgADP at the high-affinity catalytic site 1. Kd1(MgATP) and Kd1(MgADP) were an order of magnitude higher in the absence of epsilon than in its presence. In contrast, the binding affinities for MgATP and MgADP at sites 2 and 3 were similar in the epsilon-depleted and epsilon-replete enzymes, as were the affinities at all three sites for free ATP and ADP. Comparison of MgATP binding and hydrolysis parameters showed that in the presence as well as the absence of epsilon, Km equals Kd3. Thus, in both cases, all three catalytic binding sites have to be occupied to obtain rapid (Vmax) MgATP hydrolysis rates.

Amino acid substitutions in the a subunit affect the epsilon subunit of F1F0 ATP synthase from Escherichia coli

Amino acid substitutions at many positions in the a subunit of F1F0 ATP synthase result in impaired proton translocation and altered catalytic activity. In this work, we demonstrate that amino acid substitutions in the a subunit affect the epsilon subunit. In mutant F1F0 ATP synthases, the epsilon subunit was studied by determining its sensitivity to proteolysis and by chemical crosslinking under conditions of active turnover and in quiescent enzyme. Like native F1F0 ATP synthase, the epsilon subunit in enzymes carrying either the aarg-210-->ile or agly-218-->asp substitutions proved resistant to trypsin digestion during ATP hydrolysis. In each case, the epsilon subunit was rapidly digested in the presence of a nonhydrolyzable ligand, but this did not result in the activation of hydrolytic activity typically seen in wild-type enzyme. In enzyme carrying the aala-217-->arg substitution, the trypsin digestion of the epsilon subunit occurred regardless of ligand and was accompanied by a limited hydrolytic activation. Relative to the native F1F0 ATP synthase, the aala-217-->arg substitution resulted in reduced efficiency of crosslinking between the epsilon and beta subunits using 1-ethyl-3-[3-(dimethylamino)propyl]carbodiimide. These observations indicate that the structural changes resulting from amino acid substitutions in the a subunit are propagated to the epsilon subunit and are specific to the individual substitutions.

Catalytic mechanism of F1-ATPase

The structure of the core catalytic unit of ATP synthase, alpha 3 beta 3 gamma, has been determined by X-ray crystallography, revealing a roughly symmetrical arrangement of alternating alpha and beta subunits around a central cavity in which helical portions of gamma are found. A low-resolution structural model of F0, based on electron spectroscopic imaging, locates subunit a and the two copies of subunit b outside of a subunit c oligomer. The structures of individual subunits epsilon and c (largely) have been solved by NMR spectroscopy, but the oligomeric structure of c is still unknown. The structures of subunits a and delta remain undefined, that of b has not yet been defined but biochemical evidence indicates a credible model. Subunits gamma, epsilon, b, and delta are at the interface between F1 and F0; gamma epsilon complex forms one element of the stalk, interacting with c at the base and alpha and beta at the top. The locations of b and delta are less clear. Elucidation of the structure F0, of the stalk, and of the entire F1F0 remains a challenging goal.

The delta- and epsilon-subunits of bovine F1-ATPase interact to form a heterodimeric subcomplex

The delta-subunit of bovine F1-ATPase was expressed from a bacterial vector at fairly high level in Escherichia coli, but the yield of bovine epsilon-subunit was rather low under similar conditions. However, co-expression of the proteins from a dicistronic operon delta-epsilon in the same expression vector, produced both of them in good yield in a soluble form in the bacterial cytoplasm, and by chromatography it was found that the delta- and epsilon-subunits were associated in a stable complex. The amino groups in the complex were labelled exhaustively by chemical reaction under denaturing conditions with ethyl-[1-14C]acetimidate. The alpha-amino groups of the proteins were unmodified, but complete reaction of all epsilon-amino groups in both proteins was demonstrated by determination of the molecular masses of the modified proteins by electrospray MS. The modified subunits were separated by denaturing gel electrophoresis, and from measurements of the ratio of incorporated radioactivities and the lysine contents of the proteins, it was calculated that the subcomplex contains equimolar amounts of the two proteins. As the apparent molecular mass of the complex determined by gel filtration was 29 kDa, it appears that the complex contains one copy of each protein. It is likely that the delta- and epsilon subunits are associated in a similar manner in the bovine F1-ATPase complex, and that, like a bacterial homologue of the delta-subunit, they interact with the gamma- and beta-subunits.