How to achieve Tat transport with alien TatA

TatA is an essential and structurally conserved component of all known Twin-arginine transport (Tat) machineries which are able to catalyse membrane transport of fully folded proteins. Here we have investigated if bacterial TatA, or chimeric pea/E. coli TatA derivatives, are capable of replacing thylakoidal TatA in function. While authentic E. coli TatA does not show any transport activity in thylakoid transport experiments, TatA chimeras comprising the transmembrane helix (TMH) of pea TatA are fully active. For minimal catalytic activity it is even sufficient to replace three residues within TMH of E. coli TatA by the corresponding pea residues. Almost any further substitution within TMH gradually raises transport activity in the thylakoid system, while functional characterization of the same set of TatA derivatives in E. coli yields essentially inverse catalytic activities. Closer inspection of the substituted residues suggests that the two transport systems have deviating demands with regard to the hydrophobicity of the transmembrane helix.