Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 230

Type II secretion systems (T2SS) are very complex, consisting of as many as 15 different proteins (Figure 2.39). Most of these proteins are in the inner membrane and periplasm, and only 1 is in the outer membrane, where 12 of the secretin polypeptides come together to form a large β-barrel with a pore large enough to pass already folded proteins. The formation of this channel requires the participation of normal cellular lipoproteins that may become part of the structure. The secretin protein has a long N terminus that extends through the periplasm to make contact with other proteins of the T2SS in the inner membrane. This periplasmic portion of the secretin may also gate the channel, as with the TolC channel.

Even though many of the components of the T2SS are in the inner membrane, they use either the SecYEG channel or the Tat pathway to get their substrates through the inner membrane. Therefore, proteins transported by this system have either the Sec type or the Tat type of cleavable signal sequences at their N termini. Protein folding is usually completed in the periplasm before transport through the outer membrane. Some of the periplasmic and inner membrane proteins of the secretion system are related to components of pili and have been called pseudopilin proteins (see chapter 4). It has been proposed that the formation and retraction of these pseudopili work like a piston to push the protein through the secretin channel in the outer membrane to the outside of the cell. In this way, the energy for secretion could come from the inner membrane or the cytoplasm, as shown in the figure, since, as mentioned above, there is no source of energy in the periplasm. In support of this model, the pseudopili have been seen to produce pili outside an E. coli cell when the gene for the pilin-like protein was cloned and overproduced in E. coli.

Some examples of proteins secreted by T2SS are the pullulanase of Klebsiella oxytoca and the cholera toxin of Vibrio cholerae. The pullulanase degrades starch, and the cholera toxin is responsible for the watery diarrhea associated with the disease cholera (see chapter 12). The cholera toxin is composed of two subunits, A and B, and after transport by the SecYEG channel, one of the A and five of the B subunits assemble in the periplasm, followed by secretion through the secretin channel and into the intestine of the vertebrate host. The associated B subunit then assists the A subunit into mucosal cells, where it ADP-ribosylates (adds ADP) to a membrane protein that regulates the adenylate cyclase. This disrupts the signaling pathways and causes diarrhea. T2SS are also related to some DNA transfer systems used in transformation (see chapter 6) and are closely related to the systems that assemble type IV pili on the cell surface (see below).