Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 217

As mentioned above, the defining feature of proteins that are to be transported into the inner membrane or beyond by the SecYEG channel is the presence at their N termini of a signal sequence. The nature and fate of this signal sequence depend upon the ultimate destination of the transported protein. For proteins that are to be transported through the inner membrane into the periplasm and beyond, the signal sequence is approximately 20 amino acids long and consists of a basic region at the N terminus, followed by a mostly hydrophobic region and then a region with some polar amino acids. In contrast, most proteins whose final destination is the inner membrane merely use their first N-terminal transmembrane domain as a signal sequence.

If the protein is to be secreted through the membrane, the signal sequence is removed by a protease as the protein passes through the SecYEG channel (Figure 2.38B). The most prevalent of the proteases that clip off signal sequences in E. coli is the Lep protease (for leader peptide protease), but there is at least one other, more specialized protease called LspA, which removes the leader sequence from some lipoproteins destined for the outer membrane. Proteins that are destined to be transported beyond the inner membrane but have just been synthesized and so still retain their signal sequences are called presecretory proteins. When the short signal sequence is removed in the SecYEG channel, the presecretory protein becomes somewhat shorter before it reaches its final destination in the periplasm or the outer membrane or outside the cell.