The N-terminal Domain of cpTatC Protein Interacts with the Precursor Mature Domain in Chloroplast TAT Translocation

The chloroplast Twin Arginine Transport (cpTAT) protein translocation pathway is one of the thylakoid membrane's two protein transport pathways for getting proteins into the lumen. The cpTAT system distinguishes itself by transporting fully folded proteins across the thylakoid, using the sole energy source of the proton motive force (PMF). The cpTAT pathway is evolutionarily conserved with the TAT pathway found in many bacteria and archaea. Although small differences exist, TAT systems in different organisms share homologous protein composition and similar molecular mechanisms. The cpTAT system comprises cpTatC, Hcf106, and Tha4 (the prokaryotic homologs of these proteins are TatC, TatB, and TatA, respectively). (cp)TatC is one of the essential proteins in the (cp)TAT system, as it is present in the receptor complex. The amino acid sequence alignment of cpTatC and TatC protein in archaea and E. coli has shown a unique N-terminal amino acid extension of 70-100 amino acids in mature cpTatC that is not present in prokaryotic TatC. However, the role of the amino-terminal extension in cpTatC function is still unknown. We present crosslinking evidence that the amino-terminal extension directly interacts with the precursor mature domain during TAT protein translocation and may serve to prime the transporter with bound precursor in the absence of the PMF.

The cpTatC protein in the cpTAT pathway has a unique amino-terminal extension not found in bacterial or archaeal TatC proteins.The N-terminal extension of cpTatC interacts with the precursor mature domain during TAT translocation.The crosslinking data indicate that different regions within the N-terminal extension exhibit varying intensities when binding to the precursor.