Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 224

The signal sequence recognized by the Tat system is structurally similar to the signal sequence recognized by the Sec system, with a positively charged region followed by a longer hydrophobic region and a polar region, and is cleaved from the protein as it passes through the channel. However, it is somewhat longer, especially in the positively charged region. Two of the positively charged amino acids at the junction of the charged and hydrophobic regions are usually arginines, which give the Tat system its name (twin-arginine transport). The arginines are found in the motif S-R-R, although the first of the twin arginines is sometimes a lysine (K). This sequence is followed by two hydrophobic amino acids, usually F and L, and then often by a K.

The presence of this particular signal sequence at the N terminus of a newly synthesized protein targets the protein for transport by the Tat system rather than by the SecYEG channel. This raises an interesting question. How does the system know the protein has already folded properly and contains all the needed cofactors, etc., so that it is time to transport it? The Tat system needs a “quality control” system to ensure that it transports only properly folded proteins and does not transport proteins that are unfolded or only partially folded. This quality control system should also be specific for each protein to be transported, since each type of folded protein has a unique structure (see above). E. coli solves this problem by encoding dedicated proteins that specifically bind to the Tat signal sequence of only one type of protein and come off only when that protein has folded properly (see Palmer et al., Suggested Reading).