Robert H. Fillingame

The preferred stoichiometry of c subunits in the rotary motor sector of Escherichia coli ATP synthase is 10

The stoichiometry of c subunits in the H+-transporting Fo rotary motor of ATP synthase is uncertain, the most recent suggestions varying from 10 to 14. The stoichiometry will determine the number of H+ transported per ATP synthesized and will directly relate to the P/O ratio of oxidative phosphorylation. The experiments described here show that the number of c subunits in functional complexes of FoF1 ATP synthase from Escherichia coli can be manipulated, but that the preferred number is 10. Mixtures of genetically fused cysteine-substituted trimers (c3) and tetramers (c4) of subunit c were coexpressed and the c subunits crosslinked in the plasma membrane. Prominent products corresponding to oligomers of c7 and c10 were observed in the membrane and purified FoF1 complex, indicating that the c10 oligomer formed naturally. Oligomers larger than c10 were also observed in the membrane fraction of cells expressing c3 or c4 individually, or in cells coexpressing c3 and c4 together, but these larger oligomers did not copurify with the functional FoF1 complex and were concluded to be aberrant products of assembly in the membrane.

Aqueous access pathways in subunit a of rotary ATP synthase extend to both sides of the membrane

The role of subunit a in promoting proton translocation and rotary motion in the Escherichia coli F1Fo ATP synthase is poorly understood. In the membrane-bound Fo sector of the enzyme, H+ binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of subunit c. Protons are thought to reach Asp-61 at the center of the membrane via aqueous channels formed at least in part by one or more of the five TMHs of subunit a. Aqueous access pathways have previously been mapped to surfaces of aTMH4. Here we have substituted Cys into the second and fifth TMHs of subunit a and carried out chemical modification with Ag+ and N-ethylmaleimide to define the aqueous accessibility of residues along these helices. Access to cAsp-61 at the center of the membrane may be mediated in part by Ag+-sensitive residues 248, 249, 251, and 252 in aTMH5. From the periplasmic surface, aqueous access to cAsp-61 may be mediated by silver-sensitive residues 115, 116, 119, 120, 122, and 126 in aTMH2. The Ag+-sensitive residues in TMH2, -4, and -5 form a continuum extending from the periplasmic to the cytoplasmic side of the membrane. In an arrangement of helices supported by second-site revertant and crosslinking analyses, these residues cluster at the interior of a four-helix bundle formed by TMH2–5. The aqueous access pathways at the interior of subunit a may be gated by a swiveling of helices in this bundle, alternately exposing cytoplasmic and periplasmic half channels to cAsp-61 during the H+ transport cycle.

Aqueous Access Pathways in ATP Synthase Subunit a REACTIVITY OF CYSTEINE SUBSTITUTED INTO TRANSMEMBRANE HELICES 1, 3, AND 5

Subunit a is thought to play a key role in H+ transport-driven rotation of the subunit c ring in Escherichia coli F1F0 ATP synthase. In the membrane-traversing F0 sector of the enzyme, H+ binding and release occurs at Asp-61 in the middle of the second transmembrane helix (TMH) of subunit c. Protons are thought to reach Asp-61 via aqueous channels formed at least in part by one or more of the five TMHs of subunit a. Aqueous access to surfaces of TMHs 2, 4, and 5 was previously suggested based upon the chemical reactivity of cysteine residues substituted into these helices. Here we have substituted Cys into TMH1 and TMH3 and extended the substitutions in TMH5 to the cytoplasmic surface. One region of TMH3 proved to be moderately Ag+-sensitive and may connect with the Ag+-sensitive region found previously on the periplasmic side of TMH2. A single Cys substitution in TMH1 proved to be both N-ethylmaleimide (NEM)-sensitive and Ag+-sensitive and suggests a possible packing interaction of TMH1 with TMH2 and TMH3. New Ag+- and NEM-sensitive residues were found at the cytoplasmic end of TMH5 and suggest a possible connection of this region to the NEM- and Ag+-sensitive region of TMH4 described previously. From the now complete pattern of TMH residue reactivity, we conclude that aqueous access from the periplasmic side of F0 to cAsp-61 at the center of the membrane is likely to be mediated by residues of TMHs 2, 3, 4, and 5 at the center of a four-helix bundle. Further, aqueous access between cAsp-61 and the cytoplasmic surface is likely to be mediated by residues in TMH4 and TMH5 at the exterior of the four-helix bundle that are in contact with the c-ring.

Structural Interactions between Transmembrane Helices 4 and 5 of Subunit a and the Subunit c Ring of Escherichia coli ATP Synthase

Subunit a plays a key role in promoting H+ transport and the coupled rotary motion of the subunit c ring in F1F0-ATP synthase. H+ binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of F0 subunit c. H+ are thought to reach Asp-61 via aqueous pathways mapping to the surfaces of TMHs 2–5 of subunit a. TMH4 of subunit a is thought to pack close to TMH2 of subunit c based upon disulfide cross-link formation between Cys substitutions in both TMHs. Here we substituted Cys into the fifth TMH of subunit a and the second TMH of subunit c and tested for cross-linking using bis-methanethiosulfonate (bis-MTS) reagents. A total of 62 Cys pairs were tested and 12 positive cross-links were identified with variable alkyl length linkers. Cross-linking was achieved near the middle of the bilayer for the Cys pairs a248C/c62C, a248C/ c63C, a248C/c65C, a251C/c57C, a251C/c59C, a251C/c62C, a252C/c62C, and a252C/c65C. Cross-linking was achieved near the cytoplasmic side of the bilayer for Cys pairs a262C/c53C, a262C/c54C, a262C/c55C, and a263C/c54C. We conclude that both aTMH4 and aTMH5 pack proximately to cTMH2 of the c-ring. In other experiments we demonstrate that aTMH4 and aTMH5 can be simultaneously cross-linked to different subunit c monomers in the c-ring. Five mutants showed pH-dependent cross-linking consistent with aTMH5 changing conformation at lower pH values to facilitate cross-linking. We suggest that the pH-dependent conformational change may be related to the proposed role of aTMH5 in gating H+ access from the periplasm to the cAsp-61 residue in cTMH2.

Cross-linking between Helices within Subunit a of Escherichia coli ATP Synthase Defines the Transmembrane Packing of a Four-helix Bundle

Subunit a of F1F0 ATP synthase is required in the H+ transport driven rotation of the c-ring of F0, the rotation of which is coupled to ATP synthesis in F1. The three-dimensional structure of subunit a is unknown. In this study, Cys substitutions were introduced into two different transmembrane helices (TMHs) of subunit a, and the proximity of the thiol side chains was tested via attempted oxidative cross-linking to form the disulfide bond. Pairs of Cys substitutions were made in TMHs 2/3, 2/4, 2/5, 3/4, 3/5, and 4/5. Cu+2-catalyzed oxidation led to cross-link formation between Cys pairs L120C(TMH2) and S144C(TMH3), L120C(TMH2) and G218C(TMH4), L120C(TMH2) and H245C(TMH5), L120C(TMH2) and I246C(TMH5), N148C(TMH3) and E219C(TMH4), N148C(TMH3) and H245C(TMH5), and G218C(TMH4) and I248C(TMH5). Iodine, but not Cu+2, was found to catalyze cross-link formation between D119C(TMH2) and G218C(TMH4). The results suggest that TMHs 2, 3, 4, and 5 form a four-helix bundle with one set of key functional residues in TMH4 (Ser-206, Arg-210, and Asn-214) located at the periphery facing subunit c. Other key residues in TMHs 2, 4, and 5, which were concluded previously to compose a possible aqueous access pathway from the periplasm, were found to locate to the inside of the four-helix bundle.

S-Adenosyl-L-methionine decarboxylase during lymphocyte transformation: Decreased degradation in the presence of a specific inhibitor

Abstract S-Adenosyl-L-methionine decarboxylase (AmDC) activity increased 20- to 25-fold in a biphasic manner during Concanavalin A-induced lymphocyte transformation. When a potent inhibitor of this enzyme, methylglyoxal bis (guanylhydrazone), was added to transforming cultures, AmDC rapidly increased beyond the fully induced level and eventually reached a specific activity 2500 times that found in non-transformed lymphocytes. Measurements of the decline in enzyme activity in the presence of cycloheximide indicated that the half-life increased from 40 minutes to at least 20 hours during inhibitor treatment. It is likely that this change in the rate of AmDC degradation was primarily, if not solely, responsible for the large increases observed.

Subunit a Facilitates Aqueous Access to a Membrane-embedded Region of Subunit c in Escherichia coli F1F0 ATP Synthase

Rotary catalysis in F1F0 ATP synthase is powered by proton translocation through the membrane-embedded F0 sector. Proton binding and release occurs in the middle of the membrane at Asp-61 on transmembrane helix 2 of subunit c. Previously, the reactivity of cysteines substituted into F0 subunit a revealed two regions of aqueous access, one extending from the periplasm to the middle of the membrane and a second extending from the middle of the membrane to the cytoplasm. To further characterize aqueous accessibility at the subunit a-c interface, we have substituted Cys for residues on the cytoplasmic side of transmembrane helix 2 of subunit c and probed the accessibility to these substituted positions using thiolate-reactive reagents. The Cys substitutions tested were uniformly inhibited by Ag+ treatment, which suggested widespread aqueous access to this generally hydrophobic region. Sensitivity to N-ethylmaleimide (NEM) and methanethiosulfonate reagents was localized to a membrane-embedded pocket surrounding Asp-61. The cG58C substitution was profoundly inhibited by all the reagents tested, including membrane impermeant methanethiosulfonate reagents. Further studies of the highly reactive cG58C substitution revealed that NEM modification of a single c subunit in the oligomeric c-ring was sufficient to cause complete inhibition. In addition, NEM modification of subunit c was dependent upon the presence of subunit a. The results described here provide further evidence for an aqueous-accessible region at the interface of subunits a and c extending from the middle of the membrane to the cytoplasm.

Aqueous Accessibility to the Transmembrane Regions of Subunit c of the Escherichia coli F1F0 ATP Synthase

Rotary catalysis in F1F0 ATP synthase is powered by proton translocation through the membrane-embedded F0 sector. Proton binding and release occur in the middle of the membrane at Asp-61 on transmembrane helix (TMH) 2 of subunit c. Previously the reactivity of Cys substituted into TMH2 revealed extensive aqueous access at the cytoplasmic side as probed with Ag+ and other thiolate-directed reagents. The analysis of aqueous accessibility of membrane-embedded regions in subunit c was extended here to TMH1 and the periplasmic side of TMH2. The Ag+ sensitivity of Cys substitutions was more limited on the periplasmic versus cytoplasmic side of TMH2. In TMH1, Ag+ sensitivity was restricted to a pocket of four residues lying directly behind Asp-61. Aqueous accessibility was also probed using Cd2+, a membrane-impermeant soft metal ion with properties similar to Ag+. Cd2+ inhibition was restricted to the I28C substitution in TMH1 and residues surrounding Asp-61 in TMH2. The overall pattern of inhibition, by all of the reagents tested, indicates highest accessibility on the cytoplasmic side of TMH2 and in a pocket of residues around Asp-61, including proximal residues in TMH1. Additionally subunit a was shown to mediate access to this region by the membrane-impermeant probe 2-(trimethylammonium)ethyl methanethiosulfonate. Based upon these results and other information, a pocket of aqueous accessible residues, bordered by the peripheral surface of TMH4 of subunit a, is proposed to extend from the cytoplasmic side of cTMH2 to Asp-61 in the center of the membrane.

The Cytoplasmic Loops of Subunit a of Escherichia coli ATP Synthase May Participate in the Proton Translocating Mechanism

Subunit a plays a key role in promoting H+ transport and the coupled rotary motion of the subunit c ring in F1F0-ATP synthase. H+ binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of F0 subunit c. H+ are thought to reach Asp-61 via aqueous pathways mapping to the surfaces of TMHs 2–5 of subunit a based upon the chemical reactivity of Cys substituted into these helices. Here we substituted Cys into loops connecting TMHs 1 and 2 (loop 1–2) and TMHs 3 and 4 (loop 3–4). A large segment of loop 3–4 extending from loop residue 192 loop to residue 203 in TMH4 at the lipid bilayer surface proved to be very sensitive to inhibition by Ag+. Cys-161 and -165 at the other end of the loop bordering TMH3 were also sensitive to inhibition by Ag+. Further Cys substitutions in residues 86 and 93 in the middle of the 1–2 loop proved to be Ag+-sensitive. We next asked whether the regions of Ag+-sensitive residues clustered together near the surface of the membrane by combining Cys substitutions from two domains and testing for cross-linking. Cys-161 and -165 in loop 3–4 were found to cross-link with Cys-202, -203, or -205, which extend into TMH4 from the cytoplasm. Further Cys at residues 86 and 93 in loop 1–2 were found to cross-link with Cys-195 in loop 3–4. We conclude that the Ag+-sensitive regions of loops 1–2 and 3–4 may pack in a single domain that packs at the ends of TMHs 3 and 4. We suggest that the Ag+-sensitive domain may be involved in gating H+ release at the cytoplasmic side of the aqueous access channel extending through F0.

Chemical Reactivities of Cysteine Substitutions in Subunit a of ATP Synthase Define Residues Gating H+ Transport from Each Side of the Membrane

Subunit a plays a key role in coupling H+ transport to rotations of the subunit c-ring in F1Fo ATP synthase. In Escherichia coli, H+ binding and release occur at Asp-61 in the middle of the second transmembrane helix (TMH) of Fo subunit c. Based upon the Ag+ sensitivity of Cys substituted into subunit a, H+ are thought to reach Asp-61 via aqueous pathways mapping to surfaces of TMH 2–5. In this study we have extended characterization of the most Ag+-sensitive residues in subunit a with cysteine reactive methanethiosulfonate (MTS) reagents and Cd2+. The effect of these reagents on ATPase-coupled H+ transport was measured using inside-out membrane vesicles. Cd2+ inhibited the activity of all Ag+-sensitive Cys on the cytoplasmic side of the TMHs, and three of these substitutions were also sensitive to inhibition by MTS reagents. On the other hand, Cd2+ did not inhibit the activities of substitutions at residues 119 and 120 on the periplasmic side of TMH2, and residues 214 and 215 in TMH4 and 252 in TMH5 at the center of the membrane. When inside-out membrane vesicles from each of these substitutions were sonicated during Cd2+ treatment to expose the periplasmic surface, the ATPase-coupled H+ transport activity was strongly inhibited. The periplasmic access to N214C and Q252C, and their positioning in the protein at the a-c interface, is consistent with previous proposals that these residues may be involved in gating H+ access from the periplasmic half-channel to Asp-61 during the protonation step.