Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 229

Type I secretion systems (T1SS) secrete a protein directly from the cytoplasm to the outside of the cell (Figure 2.39). They are different from the other types of secretion systems and more closely related to a large family of ATP-binding cassette (ABC) transporters that export small molecules, including antibiotics and toxins, from the cell. The ABC transporters tend to be more specialized, exporting only certain molecules from the cell. To get the protein through the inner membrane, T1SS use a dedicated system that consists of two proteins, an ABC-type protein in the inner membrane and an integral membrane protein that bridges the inner and outer membranes. To get through the outer membrane, T1SS use a multiuse protein, TolC, that forms the β-barrel channel in the outer membrane. Because the TolC channel has other uses and also exports other molecules, including toxic compounds, from the cell, it is recruited to the T1SS only when the specific protein is to be secreted. When the molecule to be secreted binds to the ABC protein, the integral membrane protein recruits TolC, which then forms the β-barrel in the outer membrane. The cleavage of ATP by the ABC protein presumably provides the energy to push the secreted protein all the way through the TolC channel to the outside of the cell.

Figure 2.39 Schematic representation of the type I, II, III, and IV protein secretion systems. The examples shown are for type I (hemolysin A [HlyA] of Escherichia coli), type II (pullulanase of Klebsiella oxytoca), type III (Yop of Yersinia), and type IV (vir of Agrobacterium tumefaciens). EM, extracellular milieu; OM, outer membrane; Peri, periplasmic space; IM, inner membrane; Cyto, cytoplasm. The arrows indicate which pathways use the Sec and Tat pathways through the inner membrane. Modified from Henderson IR, Navarro Garcia F, et al, Microbiol Mol Biol Rev 68:692–744, 2004.

The classical example of a protein secreted by a T1SS is the HylA hemolysin protein of pathogenic E. coli. This toxin inserts itself into the plasma membrane of eukaryotic cells, creating pores that allow the contents to leak out. It uses a dedicated T1SS composed of HylB (the ABC protein) and HylD (the integral membrane protein). Because HylA is not transported through the inner membrane by either the SecYEG channel or the Tat system, it does not contain a cleavable N-terminal signal sequence. Instead, like all proteins secreted by T1SS, it has a sequence at its carboxyl terminus that is recognized by the ABC transporter but, unlike a signal sequence, is not cleaved off as the protein is exported.

The TolC channel has been crystallized and its structure determined (see Koronakis et al., Suggested Reading). This structure has provided interesting insights into the structure of β-barrels in general and how they can be gated and opened to transport specific molecules. Briefly, three TolC polypeptides come together to form the channel through the outer membrane. Each of these monomers contributes four transmembrane domains to form a β-barrel that is always open on one side of the outer membrane, the side on the outside of the cell. In addition, each monomer has four longer α-helical domains that are long enough to extend all the way across the periplasm. These four α-helical domains contribute to the formation of a second channel that is aligned with the first channel and traverses the periplasm. Because of these two channels, the secreted protein can be transported all the way from the inner membrane to the outside of the cell. In addition, the channel in the periplasm can open and close and therefore “gate” the channel. When a protein is being transported and the TolC channel is recruited, the α-helical domains of the periplasmic channel may rotate, which untwists them and opens the gate on the periplasmic side. The molecule is then secreted all the way through both channels to the outside of the cell.