Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 238

In bacteria that lack a bilipid outer membrane, the cell wall is available to the external surface of the cell. These bacteria take advantage of this by attaching some proteins directly to their cell wall so that the proteins will be exposed on the cell surface. Proteins destined for covalent attachment to the outer surface of the cell wall are the targets of a type of cell wall-sorting enzyme called a sortase. The general sortase, sortase A (StrA), sometimes called the housekeeping sortase, may attach as many as 40 different proteins to the cell wall. Others are more specific for certain proteins.

A sortase is able to create covalent attachments between peptides by catalyzing a transpeptidation reaction. Figure 2.42 illustrates a typical StrA pathway. Surface proteins that are StrA targets contain an N-terminal signal sequence and a 30- to 40-residue C-terminal sorting signal, which is composed of a pentapeptide cleavage site, LPXTG, and a hydrophobic domain that together constitute the cell wall-sorting signal (Cws) (Figure 2.42A). The N-terminal signal sequence of the sortase target protein directs the protein to the membrane translocase, where the signal sequence is removed (Figure 2.42B). After the protein has been translocated across the cytoplasmic membrane, the sortase cuts between the threonine (T) and glycine (G) in the pentapeptide sorting signal and then covalently links the carboxyl group of the threonine to a specific cysteine in the sortase C terminus. The sortase then attaches it to the terminal amino acid in the lipid II interlinking peptide that links two peptide crosslinks in the cell wall. When the N-acetylmuramic acid (MurNac) in this lipid II is incorporated into the cell wall, the protein becomes covalently attached to the cell wall.

Figure 2.41 Comparison of the Firmicute-type injectosome (A) and the type III injectisome (B). Modified from Tweten RK, Caparon M, in Waksman G, Caparon M, Hultgren S (ed), Structural Biology of Bacterial Pathogenesis (ASM Press, Washington, DC, 2005).

Sortase subfamilies are defined based on their taxonomic distributions and differences in the sorting signal and the peptide to which it can be attached (see Hendrickx et al., Suggested Reading). A particularly interesting example is the assembly of pili from their subunits on the cell surfaces. Pili are composed of one major subunit, often called pilin, and one or more minor subunits. In the simplest example, pili composed of pilin and only one other subunit, the minor subunit at the tip is attached to the first pilin subunit by a specialized sortase, and this first subunit is then attached to a number of other pilin subunits by the same specialized sortase. Finally, the last subunit is attached to the cell wall by the housekeeping sortase A, which attaches most other proteins to the cell wall. This makes for a very stable pilus and secures its attachment to the cell wall.