Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 215

About one-fifth of the proteins made in a bacterium do not remain in the cytoplasm and instead are transported or exported into or through the surrounding membranes. The terminology is often used loosely, but we refer to proteins that leave the cytoplasm as being transported. The process of transferring them through one or both membranes is secretion. If they are transferred through both membranes to the exterior of the cell, they are exported. Correspondingly, proteins that remain in either the inner or outer membrane are inner membrane proteins or outer membrane proteins, while those that remain in the periplasmic space are periplasmic proteins. Proteins that are passed all the way out of the cell into the surrounding environment are exported proteins.

By far the largest group of proteins that are transported from the cytoplasm are destined for the inner membrane. Inner membrane proteins often extend through the membrane a number of times and have some stretches that are in the periplasm and other stretches that are in the cytoplasm. The stretches that traverse the membrane have mostly uncharged, nonpolar (hydrophobic [see the inner cover]) amino acids, which make them more soluble in the membranes. A stretch of about 20 mostly hydrophobic amino acids is long enough to extend from one side of the bipolar lipid membrane to the other, and such stretches in proteins are called the transmembrane domains. The less hydrophobic stretches between them are called the cytoplasmic domains or periplasmic domains, depending on whether they extend into the cytoplasm on one side of the membrane or into the periplasm on the other side. Proteins with domains on both sides of the membrane are called transmembrane proteins and are very important, because they allow communication from outside the cell to the cytoplasm. Some transmemembrane proteins that play such a communicating role are discussed in chapter 12.