Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 216

Transported proteins usually contain many amino acids that are either polar or charged (basic or acidic), which makes it difficult for them to pass through the membranes. They must be helped in their translocation through the membrane by other specialized proteins. Some of these proteins form a channel in the membrane. Some transported proteins make their own dedicated channel, but most use the more general channel called the translocase, so named because its function is to translocate proteins.

A current picture of the structure of the translocase that helps proteins pass through the inner membrane, as well as how it works, is outlined in Figure 2.38. We can predict some of the features this channel must have. It must have a relatively hydrophilic inner channel through which charged and polar amino acids can pass. It also must normally be closed and should open only when a protein is passing through it; otherwise, other proteins and small molecules would leak in and out of the cell through the channel. Even the leakage of molecules as small as protons cannot be tolerated, because it would destroy the proton motive force. The channel is made up of one each of three proteins, SecY, SecE, and SecG, and is therefore called the SecYEG channel or SecYEG translocase. These three proteins form a heterotrimer made up of one each of the three different polypeptides. The SecY protein is by far the largest of the three proteins and forms the major part of the channel, while the other two proteins play more ancillary, albeit important, roles. One heterotrimer can form a large enough channel to let an unfolded protein through (see van den Berg et al., Suggested Reading), but it seems likely that more than one of these heterotrimers is involved.

Figure 2.38 Protein transport systems. (A) Cutaway view of the secretion sec channel. SecY, SecE, and SecG (not shown) form the translocase. SecY forms the channel, ring, and plug. The signal sequence of the transported protein moves the plug toward SecE. (B) Posttranslational secretion by the SecB-SecA system. SecB keeps the protein unfolded until it binds to SecA, which interacts with SecY. The signal sequence is removed, in this case by Lep protease. The exported protein is folded in the periplasm or may be secreted across the outer membrane by one of the dedicated secretion systems. (C) Cotranslational transport by the signal recognition particle (SRP) system. SRP binds to the first transmembrane domain as it emerges from the ribosome and then binds to the FtsY docking protein, bringing the ribosome to interact with SecY. The protein is translated, driving it into the SecYEG channel. The transmembrane domains of the protein somehow escape through the side of the channel into the membrane, in some cases with the help of the YidC protein, as shown.

Besides forming the major part of the channel, a region of the SecY protein forms a hydrophobic “plug,” which opens only when a protein is passing through (Figure 2.38). The binding of a signal sequence (see below) in a protein to be transported causes the plug to move over toward SecE on the side of the channel, opening the channel. As a result, only proteins that have a bona fide signal sequence can be translocated.