Figure 3.

Model of pilus biogenesis. Pilin precursors (SpaA, denoted by pink circles; SpaB, denoted by dark-aqua ovals; and SpaC, denoted by light-aqua ovals) are synthesized in the cytoplasm and translocated across the membrane by the Sec machinery (step 1). At the exoplasm, the precursors subsequently form acyl–enzyme intermediates with the housekeeping sortase (green) (step 2) or pilus-specific sortase (gray). These enzyme intermediates are capable of transferring these pilins to the lipid II precursor, thus anchoring monomeric pilins to the cell wall (step 3 in A). The pilus-specific sortase catalyzes pilus polymerization (step 4) by the mechanism described in Box 2. Pilus polymerization is terminated when pilus polymers are transferred to lipid II in one of two possible ways. In one pathway, the housekeeping sortase having a SpaA monomer would receive the pilus polymer from the pilus-specific sortase (step 5) and transfer the polymer to lipid II (step 6). In the alternative pathway (not shown), the pilus-specific sortase would transfer the polymer directly to lipid II. Red diamonds denote the D-diaminopimelic moiety of the cell wall pentapeptide. SecYEG stands for the three subunits of the general secretion machinery (Sec).