Insertion Scanning Mutagenesis of Subunit a of the F1F0 ATP Synthase near His245and Implications on Gating of the Proton Channel

Abstract

Subunit a of the E. coli F 1 F 0 ATP synthase was probed by insertion scanning mutagenesis in a region between residues Glu 219 and His 245 . A series of single amino acid insertions, of both alanine and aspartic acid, were constructed after the following residues: 225, 229, 233, 238, 243, and 245. The mutants were tested for growth yield, binding of F 1 to membranes, dicyclohexylcarbodiimide sensitivity of ATPase activity, ATP-driven proton translocation, and passive proton permeability of membranes stripped of F 1 . Significant loss of function was seen only with insertions after positions 238 and 243. In contrast, both insertions after residue 225 and the alanine insertion after residue 245 were nearly identical in function to the wild type. The other insertions showed an intermediate loss of function. Missense mutations of His 245 to serine and cysteine were nonfunctional, while the W241C mutant showed nearly normal ATPase function. Replacement of Leu 162 by histidine failed to suppress the 245 mutants, but chemical rescue of H245S was partially successful using acetate. An interaction between Trp 241 and His 245 may be involved in gating a "halfchannel" from the periplasmic surface of F 0 to Asp 61 of subunit a.The F 1 F 0 ATP synthase from Escherichia coli is typical of the ATP synthases found in mitochondria, chloroplasts, and many other bacteria (for recent reviews, see Refs. 1-4). It comprises an F 1 complex, which contains the nucleotide-binding subunits involved in catalysis, and an F 0 complex, which conducts protons across the membrane. The enzyme from E. coli appears to be a minimal form of the ATP synthase, with eight essential subunits. Five different subunits are found in F 1 : ␣, ␤, ␥, ␦, and ⑀, in a stoichiometry of 3:3:1:1:1. Three different subunits, a, b, and c, form F 0 with a likely stoichiometry of 1:2:9 -12 (5). The movement of protons through F 0 , from the periplasm to the cytoplasm, drives the net synthesis of ATP by F 1 .The pathway of protons through F 0 is thought to involve subunits a and c. Subunit b is essential for assembly of a functional F 0 (6, 7) but is not likely to participate directly in proton translocation. The subunits b are embedded in the membrane through a span of about 30 hydrophobic amino acids at the NH 2 terminus. NMR studies of c (8, 9) have confirmed the ␣-helical hairpin structure of the two predicted transmembrane spans and have provided details about the environment of the essential residue Asp 61 (10,11). Questions remain about the oligomeric structure of c subunits and how they interact with subunits a and b. Atomic force microscopy (12, 13) and electron spectroscopic imaging (14) have provided some evidence for a ring of 9 -12 c subunits adjacent to the subunits a and b. Subunit a is the largest of the F 0 subunits (271 residues). Mutagenesis has revealed that in addition to Asp 61 of subunit c, three residues in subunit a seem to be important in proton translocation: Arg 210 , Glu 219 , and His 245 (15)(16)(17)(18)(19). Of these three, only Arg 210 is...

Intragenic and intergenic suppression of the Escherichia coli ATP synthase subunit a mutation of Gly-213 to Asn: functional interactions between residues in the proton transport site

“…As expected, the cysteine replacements of residues aArg-210, aGly-218 and aGlu-219 were deleterious to enzyme function. The nearly complete loss of growth on succinate and H + pumping for the aR210C and aE219C mutants and the less severe effects of the aGly-218 mutant were consistent with results reported previously [12,[26][27][28][29]47,48].…”

“…Importantly, recent evidence now demonstrates that the c subunits also undergo ATPase-dependent rotation [15], which is consistent with several models of transport [12,[16][17][18]. Nevertheless, because biochemical and structural analyses of the Abbreviations used : ACMA, 9-amino-6-chloro-2-methoxyacridine ; CCCP, carbonyl cyanide m-chlorophenylhydrazone ; FLAG, Asp-Tyr-Lys-Asp-AspAsp-Asp-Lys ; LB, Luria-Bertani.…”

“…Furthermore, Stock et al [11] obtained a 3.9 A / resolution map of the partial mitochondrial complex from yeast, which included a decamer of c subunits arranged in the predicted ring [6,[12][13][14] with the C-terminal helix on the perimeter. Although the resolution did not permit the assignment of side chains, the electron densities of the yeast map were similar to the hairpin structure solved by NMR.…”

“…Significantly, swapping the Arg with aGln-252 to create a double mutant, aR210Q\aQ252R, retains a small amount of activity, suggesting that the position of aGln-252 is very close to that of aArg-210 [28]. Several amino acid replacements of aGln-252 by itself also cause perturbations of the transport mechanism [12,28,30]. Mutational studies have also shown functional interaction between a and c subunits.…”

“…A critical question is whether cAsp-61 and aArg-210 interact physically and functionally, as has been suggested in many models for transport [10,12,16,29]. This is a difficult question to answer because all substitutions of aArg-210 are null mutations.…”