Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 219

Proteins that have a removable signal sequence and are transported through the inner membrane into the periplasm or beyond are most often targeted by the SecB system in E. coli and the other bacteria that have it. The SecB protein is a specialized chaperone that binds to presecretory proteins either cotranslationally (e.g., as soon as the N-terminal region of the polypeptide emerges from the ribosome) or after they are completely synthesized, thereby preventing them from folding prematurely and ensuring that the signal sequence is exposed. The SecB chaperone passes the unfolded protein to SecA, which facilitates the association of the protein with the SecYEG channel, perhaps by binding simultaneously to the signal sequence and to a SecYEG heterotrimer (Figure 2.38B). Some bacteria have paralogs of SecA that may be dedicated to transporting only one or very few proteins. After SecA binds to the channel, the cleavage of ATP to ADP on SecA provides the energy to drive the protein into the channel, aided by the proton motive force of the membrane. As the protein passes through the channel, it loses its signal sequence, as shown in the figure. SecB is not an essential protein, and the cell can use DnaK or other general chaperones as substitutes for SecB to help transport some proteins.