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

Siegfried M. Musser, Steven M. Theg
2000 Biochemistry  
The thylakoid transmembrane ∆pH is the sole energy source driving translocation of precursor proteins by the ∆pH/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 ∆pH is virtually unaffected from pH 6.5 to 9.2, the loss in import
more » ... oss 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 protontransfer 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.
doi:10.1021/bi000115f pmid:10889030 fatcat:giondw5oujfrpj7sxy4ep6zlea