Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 220

The SRP pathway in bacteria generally targets proteins that are to remain in the inner membrane. It consists of a particle (the SRP) made up of both a small 4.5S RNA, encoded by the ffs gene, and at least one protein, Ffh, as well as a specific receptor on the membrane, called FtsY in E. coli, to which the SRP binds. FtsY is sometimes referred to as the docking protein because it “docks” proteins targeted by the SRP pathway to the SecYEG channel in the membrane. The ftsY (filament temperature-sensitive Y) gene was originally identified through temperature-sensitive mutations that cause E. coli not to divide properly and to form long filaments of many cells linked end to end at higher temperatures, but its role in cell division is indirect.

Figure 2.38C illustrates how the SRP system works. The SRP binds to the first hydrophobic transmembrane sequence of an inner membrane protein as this region of the protein emerges from the ribosome. The complex binds to the membrane, and synthesis of the protein continues, feeding the protein directly into the SecYEG translocon as the protein emerges from the ribosome. The energy of translation due to cleavage of GTP to GDP drives the polypeptide out of the ribosome into the SecYEG translocon, replacing the role of SecA, although the SecA protein might still be required for transmembrane proteins with long periplasmic domains.

The process of translating a protein as it is inserted into the translocon is called cotranslational translocation. There is a good reason why proteins destined for the inner membrane are cotranslated with their insertion into the translocon in the membrane while proteins targeted by the SecB pathway can first be translated in their entirety and then inserted into the translocon. Inner membrane proteins are much more hydrophobic than exported proteins and would form an insoluble aggregate in the aqueous cytoplasm if they were translated in their entirety before being transported into the membrane (see Lee and Bernstein, Suggested Reading).

What happens after an inner membrane protein enters the SecYEG channel is less clear. The transmembrane domains of the protein must escape the SecYEG channel and enter the surrounding membrane, while the periplasmic and cytoplasmic domains must stay in the correct compartments. Presumably, the SecYEG channel has a lateral gate that opens and allows the transmembrane domains of the protein to escape into the membrane. Another inner membrane protein called YidC might help in this process (Figure 2.38C) (see Xie and Dalbey, Suggested Reading). YidC seems to be required for the lateral escape of some proteins but not others. Some inner membrane proteins bypass SecYEG altogether and require only YidC to enter the inner membrane.