Proton Transfer Limits Protein Translocation Rate by the Thylakoid ΔpH/Tat Machinery

Abstract

The thylakoid transmembrane DeltapH is the sole energy source driving translocation of precursor proteins by the DeltapH/Tat machinery. Consequently, proton translocation must be coupled to precursor translocation. For the precursor of the 17 kDa protein of the oxygen-evolving complex (pOE17), the protein translocation process is characterized by a steep drop in efficiency at an external pH below 7.0 and above 8.7. As the membrane DeltapH is virtually unaffected from pH 6.5 to 9.2, the loss in import efficiency is a consequence of the titration of multiple residues within the translocation machinery. Transport is retarded by a factor of 2-3 in deuterium oxide (D(2)O) relative to water, strongly suggesting that proton-transfer reactions limit translocation rate. The solvent isotope effect manifests itself after the precursor binds to the membrane, indicating that the rate-limiting step is a later event in the transport process.

Twin-arginine-dependent translocation of folded proteins

“…This result could be explained by assuming that both components of the PMF, the DpH and the Dc, can contribute equally to the driving force of the Tat pathway as has been experimentally verified [144]. As a consequence of the equivalency of DpH and Dc, a coupling of H þ -flow and protein transport would have to be postulated [145], which is in good accordance with previous studies by Theg's group [146,147].…”

Compensatory role of PspA, a member of the phage shock protein operon, in rpoE mutant Salmonella enterica serovar Typhimurium

“…More recently, induction of PspA protein synthesis was shown in E. coli lacking the twin‐arginine translocation (Tat) pathway (DeLisa et al ., 2004), although others (Jones et al ., 2003) have not observed increased pspA expression upon loss of the Tat pathway. It should be noted that protein translocation via the Sec pathway requires ATP hydrolysis, while the Tat pathway is dependent on PMF (Musser and Theg, 2000).…”

The Bacterial Twin-Arginine Translocation Pathway

“…For comparison, Secdependent translocation is less energetically costly and has been estimated to require one ATP molecule per 20 amino acyl residues of the substrate translocated (130). A reduction in Tatdependent transport was observed in the presence of D 2 O relative to H 2 O, suggesting that proton transfer is involved at some stage, perhaps via a H+/protein counterflux principle (87).…”