Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 228

At least six basic types of protein secretion systems, imaginatively named types I to VI, have been identified. All of these secretion systems rely on channels in the outer membrane (called β-barrels or secretins) formed from β-sheets organized in a ring (see Figure 2.20 for an explanation of protein secondary and tertiary structures). The β-barrels are assembled so that the side chains of charged and polar amino acids tend to be in the center of the barrel, where they are in contact with hydrophilic proteins that are passing through, while the side chains of hydrophobic amino acids are on the outside of the barrel in contact with the very hydrophobic surrounding membrane. Assembly of the β-barrels requires a complex of proteins called the Bam complex (BamA, -B, -C, etc.) and periplasmic chaperones, including Skp.

Having channels in the outer membrane presents some of the same problems associated with having channels in the cytoplasmic membrane, such as the SecYEG channel. For example, how do they select the proteins that are to go through without letting others through, and how do they keep smaller molecules from going in and out? This process is called channel gating; the gate is open only when the protein being exported passes through. A second issue is the source of the energy to export a protein through the outer membrane. There is no ATP or GTP in the periplasmic space to provide energy, and the outer membrane is not known to have a proton gradient across it to create an electric field. In this section, we describe mechanisms used by the various secretion systems for solving these problems and mention some examples of proteins exported by each of the systems.