Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 234

Chaperone-usher secretion is related to type V secretion. This type of secretion is often used to assemble some types of pilins on the cell surface, such as the P pilus of uropathogenic E. coli. The secretion system consists of three proteins, a β-barrel-forming protein in the outer membrane called the usher, a periplasmic protein called the chaperone, and the pilin subunit to be assembled on the cell surface. The pilin protein is transported through the inner membrane by the SecYEG channel and therefore has a cleavable signal sequence. Once in the periplasm, the pilin protein is bound by the dedicated periplasmic chaperone. However, rather than merely helping it fold like other chaperones, this chaperone actually contributes a strand to the pilus protein that completes one of the folds of the pilin protein and makes it much more stable (see Waksman and Hultgren, Suggested Reading). The complex of pilin protein and chaperone is then targeted to the usher channel in the outer membrane, where it is assembled into the growing pilus. Again, there is the problem of where the energy for pilus assembly comes from, since the assembly of the pilus occurs at the inner face of the outer membrane after the pilin protein has been transported through the inner membrane and cytoplasm. One idea is that the periplasmic chaperone holds the pilin protein in a high-energy state, and its eventual folding at the usher drives the assembly process.