Aqueous Access Pathways in ATP Synthase Subunit a

Abstract

؉ -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 F 0 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.

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

“…The reactivity of a substituted cysteine to thiolate-directed probes provides an indication of aqueous accessibility because the reactive thiolate species is preferentially formed in an aqueous environment. The aqueous accessibility of the five TMHs in subunit a of E. coli F 0 has been probed using Ag ϩ and NEM (19,(25)(26)(27). The results suggest the presence of an aqueous accessible channel in subunit a in the center of TMHs 2-5 extending from the periplasm to the center of the membrane.…”

“…The first model proposes that the ions bound at Asp-61 exit to the cytoplasm via a half-channel composed at least partially by residues in TMH4 of subunit a (25)(26)(27). Chemical modification studies of Cys-substituted subunit a of E. coli revealed an aqueous accessible surface of TMH4 that includes the essential Arg-210 residue, which extended from the center of the membrane to the cytoplasm, suggesting that the ion exit channel may lie at the a-c interface (19,25).…”

The proximal N-terminal amino acid residues are required for the coupling activity of the bovine heart mitochondrial factor B

“…The role of bacterial subunit a charged residue Arg210 is thought to prevent the short-circuiting of both channels and to facilitate deprotonation of Asp61 at the mouth of exit half channel [41,42].…”

The ATP Synthase a-subunit of Extreme Alkaliphiles Is a Distinct Variant

“…Essential steps in coupling of ATP synthesis to the PMF include the protonation of successive c-subunits of the rotor and, after full rotation of a protonated c-subunit, de-protonation of that subunit through interactions of c-subunits of the rotor with the a-subunit stator component (4,5,7). No high resolution structural data are yet available for the a-subunit, but extensive biochemical and genetic evidence indicates that this ATP synthase subunit plays roles in providing the proton path from outside the membrane surface to the carboxylates of interacting c-subunits of the rotor (4,(17)(18)(19)(20)(21)(22)(23)(24). An essential, conserved arginine in TMH4 (Arg-210 in Escherichia coli) is proposed to prevent proton short-cutting to the cytoplasm without rotation (25) and to cause a shift in the pK a of the essential carboxylate so that the proton that has completed rotation dissociates and enters the proton exit pathway leading to the cytoplasm.…”

“…This residue is located in the TMH4 in the putative proton uptake pathway of the a-subunit. TMH4 also contains the conserved, functionally critical arginine (Arg-172 in B. pseudofirmus OF4) in the proton pathway through the a-subunit (17,18,22). The unusual Lys-180 substitutes for a glycine residue typically found at this position in non-alkaliphiles (Fig.…”