Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 232

Type IV secretion systems (T4SS) utilize a secretin-like protein (VirB9) that forms a β-barrel channel in the outer membrane and extends into the periplasm, where it makes contact with proteins in the inner membrane. However, unlike true secretins, it seems to require another outer membrane protein, VirB7, to make a channel. The VirB9 protein is covalently attached to the VirB7 protein, which in turn is covalently attached to the lipid membrane, making the structure very stable. A coupling protein (VirD4) binds specific proteins and targets them to the channel. The energy for secretion comes from the cleavage of ATP or GTP in the cytoplasm by channel-associated proteins (Figure 2.39).

T4SS are discussed in chapters 5 and 6, because they are also involved in DNA transfer during conjugation and transformation. The T-DNA transfer system of Agrobacterium tumefaciens has served as the prototype T4SS and is the one about which the most is known and to which all others are compared. Accordingly, the genes and proteins of other T4SS are numbered after their counterparts in the T-DNA transfer system, named the vir genes because of their role in virulence in plants. Some of the genes in the T-DNA transferred into plant cells cause growth of the plant cell, leading to the formation of tumors called crown galls. Others trigger the plant cells to make unusual compounds, called opines, that can be used by the bacterium as a carbon, nitrogen, and energy source (see Box 5.1). Like other T4SS, the Agrobacterium system also directly injects proteins into the plant cell, which makes it a bona fide protein secretion system.