Reconstitution of the F1-ATPase activity from purified alpha, beta, gamma and delta or epsilon subunits with glutathione S-transferase fused at their amino termini

ATP Synthesis by Oxidative Phosphorylation

The F1F0-ATP synthase (EC 3.6.1.34) is a remarkable enzyme that functions as a rotary motor. It is found in the inner membranes of Escherichia coli and is responsible for the synthesis of ATP in response to an electrochemical proton gradient. Under some conditions, the enzyme functions reversibly and uses the energy of ATP hydrolysis to generate the gradient. The ATP synthase is composed of eight different polypeptide subunits in a stoichiometry of α3β3γδεab2c10. Traditionally they were divided into two physically separable units: an F1 that catalyzes ATP hydrolysis (α3β3γδε) and a membrane-bound F0 sector that transports protons (ab2c10). In terms of rotary function, the subunits can be divided into rotor subunits (γεc10) and stator subunits (α3β3δab2). The stator subunits include six nucleotide binding sites, three catalytic and three noncatalytic, formed primarily by the β and α subunits, respectively. The stator also includes a peripheral stalk composed of δ and b subunits, and part of the proton channel in subunit a. Among the rotor subunits, the c subunits form a ring in the membrane, and interact with subunit a to form the proton channel. Subunits γ and ε bind to the c-ring subunits, and also communicate with the catalytic sites through interactions with α and β subunits. The eight subunits are expressed from a single operon, and posttranscriptional processing and translational regulation ensure that the polypeptides are made at the proper stoichiometry. Recent studies, including those of other species, have elucidated many structural and rotary properties of this enzyme.

A novel kinesin-like protein, KIF1Bbeta3 is involved in the movement of lysosomes to the cell periphery in non-neuronal cells

The kinesin superfamily protein, KIF1Bbeta, a splice variant of KIF1B, is involved in the transport of synaptic vesicles in neuronal cells, and is also expressed in various non-neuronal tissues. To elucidate the functions of KIF1Bbeta in non-neuronal cells, we analyzed the intracellular localization of KIF1Bbeta and characterized its isoform expression profile. In COS-7 cells, KIF1B colocalized with lysosomal markers and expression of a mutant form of KIF1Bbeta, lacking the motor domain, impaired the intracellular distribution of lysosomes. A novel isoform of the kinesin-like protein, KIF1Bbeta3, was identified in rat and simian kidney. It lacks the 5th exon of the KIF1Bbeta-specific tail region. Overexpression of KIF1Bbeta3 induced the translocation of lysosomes to the cell periphery. However, overexpression of KIF1Bbeta3-Q98L, which harbors a pathogenic mutation associated with a familial neuropathy, Charcot-Marie-Tooth disease type 2 A, resulted in the abnormal perinuclear clustering of lysosomes. These results indicate that KIF1Bbeta3 is involved in the translocation of lysosomes from perinuclear regions to the cell periphery.

Calcineurin homologous protein isoform 2 (CHP2), Na+/H+ exchangers-binding protein, is expressed in intestinal epithelium

The Na+/H+ exchangers (NHEs) comprise a family of membrane proteins that catalyze the electroneutral exchange of Na+ and H+. Calcineurin homologous protein (CHP) acts as a crucial cofactor for NHE activity through direct interaction with the carboxyl-terminal tail region of NHEs. We have cloned a new rat CHP isoform (rCHP2) and characterized the binding property to NHEs and the tissue distribution. rCHP2 binds to the juxtamembrane region of plasma membrane-type NHE isoforms (NHE1-5) in vivo and in vitro as well as rCHP1 (original rat CHP). Interestingly, CHP2 is predominantly expressed in the small and large intestine although rCHP1 shows relatively ubiquitous expression at both the mRNA and protein levels. In situ hybridization experiments demonstrated the abundant expression of CHP2 in the epithelial cell layer of villi of the small intestine in contrast with the expression of CHP1 in both the epithelial layer and connective tissues. These results suggest that CHP2 functions in the absorptive epithelium for the intestine with NHE(s).

Reconstitution of F1-ATPase activity from Escherichia coli subunits alpha, beta and subunit gamma tagged with six histidine residues at the C-terminus

An engineered gamma subunit of Escherichia coli F1-ATPase with extra 14 and 20 amino acid residues at the N- and C-termini (His-tag gamma), respectively, was overproduced in E. coli and purified. Six histidines are included in the C-terminal extension. The reconstituted F1 containing alpha, beta, and His-tagged gamma exhibited sixty percent of the wild-type ATPase activity. The reconstituted alphabeta His-tag gamma complex was subjected to affinity chromatography with nickel-nitrilotriacetic acid (Ni-NTA) agarose resin. ATPase activity was eluted specifically with imidazole. These results implied that the tag sequence protruded to the surface of the complex and did not seriously impair the activity. The reconstituted alphabeta His-tag gamma complex, even after its binding to the resin, exhibited ATPase activity suggesting that the gamma subunit, when fixed to a solid phase, may rotate the alphabeta complex. This system may provide a new approach for analysis of the rotation mechanisms in F1-ATPase.

Subunit interactions of Escherichia coli F1-ATPase: mutants of the gamma subunits defective in interaction with the epsilon subunit isolated by the yeast two-hybrid system

Previously, we established a method to detect subunit interactions of F1-ATPase by the yeast two-hybrid system (Moritani, C., et al. Biochim. Biophys. Acta 1274, 67-72, 1996). Here, we isolated mutants of the gamma subunits defective in interaction with the epsilon subunit by this new procedure to study the molecular basis of coupling mechanisms of the F1F0-ATPase. Based on the intensities of the reporter gene expression in this system, five mutants of the gamma subunit with different levels of gamma-epsilon interactions were isolated and their single base substitutions were determined. Mutants with a substitution of Pro-55 for Leu, Thr-102 for Met, Val-141 for Asp, or Gln-235 for Leu exhibited decreased reporter gene expression, suggesting decreased levels of interaction, while Asp-85 for Gly mutation caused a higher level of expression, suggesting increased interaction. Among these point mutations, G85D, M102T, or D141V mutations were introduced into the gamma subunit gene in the plasmid carrying whole unc operon. Transformants carrying a deletion mutant of the whole unc operon with these expression plasmids were analyzed. Mutations M102T and D141V with decreased gamma-epsilon interaction caused increases of membrane-bound F1-ATPase activity and proton pumping activity, while G85D with increased gamma-epsilon interaction exhibited lower levels of F1-ATPase activity in the membranes. Molecular assembly of the F1 subunits on the mutant membranes detected by Western blotting exhibited no defect for all three mutants. These results suggested that the correlation between the ATPase activity and gamma-epsilon interaction is reciprocal and this interaction may regulate the ATPase activity. The topological and functional importance of Gly-85, Met-102, and Asp-141 together with Leu-55 and Leu-235 in gamma-epsilon interaction is discussed.

Interaction of the delta and b subunits contributes to F1 and F0 interaction in the Escherichia coli F1F0-ATPase

Interactions of the F1F0-ATPase subunits between the cytoplasmic domain of the b subunit (residues 26-156, bcyt) and other membrane peripheral subunits including alpha, beta, gamma, delta, epsilon, and putative cytoplasmic domains of the a subunit were analyzed with the yeast two-hybrid system and in vitro reconstitution of ATPase from the purified subunits as well. Only the combination of bcyt fused to the activation domain of the yeast GAL-4, and delta subunit fused to the DNA binding domain resulted in the strong expression of the beta-galactosidase reporter gene, suggesting a specific interaction of these subunits. Expression of bcyt fused to glutathione S-transferase (GST) together with the delta subunit in Escherichia coli resulted in the overproduction of these subunits in soluble form, whereas expression of the GST-bcyt fusion alone had no such effect, indicating that GST-bcyt was protected by the co-expressed delta subunit from proteolytic attack in the cell. These results indicated that the membrane peripheral domain of b subunit stably interacted with the delta subunit in the cell. The affinity purified GST-bcyt did not contain significant amounts of delta, suggesting that the interaction of these subunits was relatively weak. Binding of these subunits observed in a direct binding assay significantly supported the capability of binding of the subunits. The ATPase activity was reconstituted from the purified bcyt together with alpha, beta, gamma, delta, and epsilon, or with the same combination except epsilon. Specific elution of the ATPase activity from glutathione affinity column with the addition of glutathione after reconstitution demonstrated that the reconstituted ATPase formed a complex. The result indicated that interaction of b and delta was stabilized by F1 subunits other than epsilon and also suggested that b-delta interaction was important for F1-F0 interaction.

Bacterial expression and purification of recombinant Plasmodium yoelii circumsporozoite protein

We report the expression and purification of recombinant rodent malarial Plasmodium yoelii circumsporozoite surface protein (PyCSP) in Escherichia coli. To facilitate purification of the recombinant protein, the PyCSP was expressed as an amino-terminal fusion protein to glutathione S-transferase and as a carboxy-terminal fusion protein to a hexahistidyl tag. The expression of the fusion protein was controlled by the inducible tac promoter. Under optimal conditions the immunoreactive PyCSP represented approximately 0.04% of the total cell lysate. Western blot analysis probing with an anti-PyCSP antibody revealed a wide array of immunoreactive bands. Material isolated by affinity purification on glutathione-Sepharose 4B resin also contained multiple bands indicative of premature termination or carboxyl-terminal degradation. Analysis of protein retained on a nickel nitrilotriacetic acid resin revealed evidence of amino-terminal deleted material. Combining the two mild affinity purifications resulted in isolation of a single immunoreactive protein of approximate molecular weight of 96 kDa. We anticipate that the approach described in this study will facilitate the production of highly purified recombinant proteins.

Escherichia coli F1-ATPase subunit interactions: beta and gamma subunit peptides inhibit in vitro reconstitution of the active alpha beta gamma complex

For biochemical analysis of subunit interactions in the proton-translocating ATPase, a new approach with in vitro reconstitution of the Escherichia coli alpha beta gamma complex and the peptides derived from the subunits was established. Various portions of the beta or gamma subunits were used for in vitro reconstitution of the alpha beta gamma complex from the purified subunits. For the beta subunits, peptides corresponding to residues 226-459, 254-459, and 226-365 inhibited reconstitution, while those corresponding to residues 1-105, 1-146, and 295-459 did not. For the gamma subunits, peptides corresponding to residues 1-192 and 74-286 exhibited inhibitory effect on reconstitution, but the peptide containing residues 191-286 did not. Only inhibitory peptides blocked the assembly of the alpha beta gamma complex which was detected by nondenaturing polyacrylamide gel electrophoresis. These inhibitory peptides bound to the alpha or beta subunit on the filter, but the noninhibitory peptides did not. These results suggested that regions beta 254-294 and gamma 74-190 have sequences important for subunit interactions which interfered with those in the reconstitution mixtures. Based on comparison between X-ray crystallographic data of bovine alpha beta gamma complex and the present results, we discussed here the significance of the biochemical approach adopted in this study.

Catalytic and structural importance of Gly-454, Tyr-455, and Leu-456 in the carboxy-terminal region of Escherichia coli F1-ATPase alpha subunit

Monoclonal antibody alpha110 recognizes Leu-456 in the alpha subunit of the Escherichia coli F1-ATPase. Binding of this antibody to the alpha subunit or mutation of this residue to Pro caused enhancement of the ATPase activity, suggesting that this residue is involved in the catalytic mechanism of this molecule (H. Kanazawa et al. (1995) Arch. Biochem. Biophys. 317, 348-356). Leu-456 together with Gly-454 and Tyr-455 are the only residues in the carboxy-terminal 75 amino acids conserved among various species, suggesting that these three residues play important roles in catalysis by the ATPase. Here, we introduced site-directed mutations into these residues. Not only L456P but also G454L, Y455K, Y455L, and L456N mutations caused enhancement of the ATPase activity. Surprisingly, Y455V, L456H, and L456S caused assembly defects of F1 subunits on the membrane. Reconstitution of the alpha betagamma complex from the wild-type beta and gamma subunits with the mutant alpha subunit (L4gamma6P) exhibited enhanced ATPase activity. Addition of delta or epsilon fused to glutathione S-transferase which are functionally similar to the delta and epsilon subunits, respectively, to the reconstituted F1-ATPase did not cause significant enhancement of its activity. Decreased interaction between alpha and beta subunits with the L456P mutation was detected by the yeast two-hybrid system. According to the deduced three-dimensional structure of the bovine a subunit, Leu-456, Gly-454, and Tyr-455 are included in a small alpha helix. These results suggest that this alpha helix affects interaction of the alpha subunit with the beta subunit but not with delta or epsilon, which may be important for the catalytic mechanism and F1 assembly.

The ATP synthase--a splendid molecular machine

An X-ray structure of the F1 portion of the mitochondrial ATP synthase shows asymmetry and differences in nucleotide binding of the catalytic beta subunits that support the binding change mechanism with an internal rotation of the gamma subunit. Other structural and mutational probes of the F1 and F0 portions of the ATP synthase are reviewed, together with kinetic and other evaluations of catalytic site occupancy and behavior during hydrolysis or synthesis of ATP. Subunit function as related to proton translocation and rotational catalysis is considered. Physical demonstrations of the gamma subunit rotation have been achieved. The findings have implications for other enzymatic catalyses.