Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 222

Only proteins that have not yet folded and so are still long, flexible polypeptides can be transported by the narrow SecYEG channel. Once folded into their final three-dimensional structure, proteins are much too wide to fit through the channel. However, some proteins must fold in the cytoplasm before they can be transported. These include membrane proteins that contain redox factors, such as molybdopterin and FeS clusters, which are synthesized in the cytoplasm and can be inserted into the protein only after it has been folded. Other examples of proteins that are transported after they are folded are some heterodimers in which only one member has a signal sequence, so the other partner would be left behind if the signal sequencecontaining polypeptide is transported before the two polypeptides have combined and folded. Folded proteins and complexes are transported by the Tat system.