Both Gram-negative and Gram-positive bacteria utilize macromolecular complexes known as pili that are anchored to the cell surface for adherence to host cells and abiotic surfaces. Gram-negative pilus assembly has been very well characterized for some systems, but the discovery of Gram-positive pili is much more recent (Lauer et al., 2005). Here we discuss sortase-dependent pilus assembly in Gram-positive bacteria, assembly of chaperone-usher pathway (CUP) pili via the canonical pathway in Gram-negative bacteria, and a representative pathway of type IV pili assembly from Gram-negative bacteria. These assembly pathways are modeled in Fig. 3.1.
Figure 3.1 Examples of adhesive fiber formation by three different mechanisms. Schematic depicts three different pathways by which adhesive fibers are placed onto the surface of bacteria. (a) Sortase-dependent pilus biogenesis in Gram-positive bacteria: Schematic depicts the model describing sortase-dependent pilus biogenesis as it has been defined for Enterococcus faecalis Ebp pili (Nielsen et al., 2013). Pilin subunits are escorted across the membrane by the SecYEG translocon. There are three Ebp pilin subunits, EbpA, EbpB, and EbpC that comprise the tip adhesion, anchor, and backbone subunits, respectively. Upon translocation, a pilin-specific sortase recognizes a specific sorting signal at the C-terminus of each protein and facilitates folding of the pilin subunit, as well as cleavage of the G within the sorting signal.
In the case of E. faecalis, the tip adhesin EbpA harbors the signal LPETG recognized by the pilin-specific sortase. The EbpC backbone pilin contains the sorting signal LPSTG. All tip and backbone pilin subunits will undergo this interaction with the pilin-specific sortase. Polymerization of the pilin subunits occurs through sortase transpeptidation of the most recently integrated pilin subunit and the next pilin subunit being incorporated. When the anchor pilin is incorporated, its sorting signal undergoes cleavage of its sorting signal by the housekeeping
Translocation of CUP (Gram-negative bacteria) and sortase-dependent (Gram-positive bacteria) pilin subunits across the membrane occurs via the Sec translocon machinery (Silhavy et al., 2010).
The first sortase-dependent pilus system in Gram-positive bacteria was discovered in Corynebacterium diptheriae (Ton-That and Schneewind, 2003). These pili comprise the tip, backbone, and anchor pilins. Each of the pilin subunits contains a cell wall sorting signal located at the C-terminus. After translocation (Fig. 3.1(a)), a pilus-specific sortase enzyme cleaves the sorting signal. The pilus-specific sortase also mediates the linkage of pilin subunits to each other, and eventually, to the housekeeping sortase that anchors the pilus to the peptidoglycan cell wall (Fig. 3.1(a)) (Marraffini et al., 2006; Danne and Dramsi, 2012). Examples of bacteria harboring sortase-dependent pili are S. pneumoniae and Enterococcus faecalis. In S. pneumoniae, the Rrg pili, comprising the anchor RrgC, multiple copies of the shaft RrgB, and the adhesin RrgA, facilitate adherence to the host cell (Hilleringmann et al., 2008, 2009; Nelson et al., 2007). The endocarditis- and biofilm-associated pili (Ebp) harbored by E. faecalis have proven to be critical for adherence and biofilm formation on venous and urinary catheters (Singh et al., 2007; Nielsen et al., 2013). EbpA serves as the adhesin, EbpC is the sheath polymer, and EbpB is the anchor subunit (Nallapareddy et al., 2006, 2011).
Gram-negative bacteria, more specifically Enterobacteriaceae such as E. coli, Pseudomonas, Klebsiella, and Burkholderia species, utilize the chaperone-usher pathway (CUP) to elaborate multisubunit pili with adhesive properties, primarily
sortase A (SrtA). SrtA is the sortase responsible for facilitating the anchoring of proteins to
the cell wall. SrtA will facilitate a series of chemical reactions, including transpeptidations and transglycosylations, with the anchor pilin and lipid II to anchor the pilus to the cell wall. (b) Chaperone-usher pathway (CUP) pilus biogenesis: Modeled here is the assembly of the CUP type 1 pili in E. coli. All pilin subunits are transported into the periplasm via the Sec translocon. FimC serves as the chaperone protein to bind, fold, and transport pilus subunits through the periplasm to the FimD usher in the outer membrane. FimD facilitates the transport of pilus subunits to the exterior surface of the cell and pilus elaboration in a helical fashion. The tip adhesin FimH serves at the starting point for pilus elaboration followed by FimF and FimG. Pilus assembly continues with >1000 copies of the major type 1 pili subunit FimA incorporated in the growing pilus. For a comprehensive review of this process, please refer to the review by Waksman and Hultgren (2009). (c) Type IV pilus biogenesis: Schematic depicts a model representative of type IV pili biogenesis in Gram-negative bacteria, though both Gram-positive and Gram-negative bacteria can both produce type IV pili. Here, the model depicts biogenesis of type IV pili in V cholerae (Frans et al., 2013). PilC serves as the tip adhesin, and PilA is the major pilin subunit with hundreds to thousands of copies making up each pilus. The pilin subunits are translocated across the inner membrane via the Sec translocon and folded by the peptidase, PilD. Once folded, the PilG inner membrane protein allows for the assembly of the pilus in a helical manner that is driven by the ATPase PilB (also named PilF). The pilus begins amassing in the periplasm until it reaches the outer membrane, where it is shuttled through the secretin PilQ that forms a channel and allows the protrusion of the pilus onto the cell surface. The PilT ATPase drives disassembly and retraction of the pilus (Craig and Li, 2008). For a comprehensive review on the biogenesis of type IV pili, please refer to Ayers et al. (2010), Burrows (2005), and Giltner et al. (2012). IM, inner membrane; LPS, lipopolysaccharide; OM, outer membrane; PG, peptidoglycan; PP, periplasm.
mediated by the tip adhesins (Fronzes et al., 2008; Busch and Waksman, 2012; Chen et al., 2009; Dodson et al., 2001). CUP pilus biogenesis takes its name from the requirement for a periplasmic “chaperone” protein that interacts with pilin subunits after their translocation to the periplasm. The chaperone’s role is to stabilize pilin folding and escort pilin subunits to the outer membrane—“usher” (Fig. 3.1(b)). The pilus subunits are taken up by their cognate periplasmic chaperones as soon as they exit the Sec machinery. Pilus subunits are characterized by an incomplete immunoglobulin-like fold, which lacks the C-terminal beta-strand (Waksman and Hultgren,
2009). This incomplete fold is stabilized by the interaction of one of the chaperone strands, called the G1 strand that is “donated” by the chaperone during chaperone- subunit interaction (Waksman and Hultgren, 2009). This process is termed “donor strand complementation.” The usher is a pilus-specific component located in the outer membrane that facilitates the assembly of the pilus on the surface of the bacterial cell (Waksman and Hultgren, 2009; Geibel and Waksman, 2014). The elaboration of type 1 pili via the CUP is illustrated in Fig. 3.1(b).
In UPEC and Klebsiella pneumoniae, type 1 pili mediate adherence to manno- sylated uroplakin receptors on the bladder epithelial surface (Martinez et al., 2000; Eto et al., 2007; Thankavel et al., 1997; Zhou et al., 2001; Thumbikat et al., 2009; Bouckaert et al., 2005; Hung et al., 2002; Struve et al., 2008, 2009). In UPEC, P pili are crucial for binding to digalactoside receptors found on renal epithelial cells (Kuehn et al., 1992; Hultgren et al., 1989). The increase in sequenced E. coli genomes has revealed the presence of numerous uncharacterized CUP pili in each of the UPEC isolates evaluated (Chen et al., 2006; Welch et al., 2002). In each of these CUP pili, the tip adhesin is predicted to have specificity for a different receptor or moiety, suggesting an expanded repertoire of surfaces that UPEC can adhere to in the environment and within the host. K. pneumoniae also express type 3 CUP pili, which have been shown to play a role in adherence to cells and silicone catheters (Di Martino et al., 2003; Murphy et al., 2013).
In addition to CUP pili, type IV pili are also well characterized and fairly ubiquitous among the culturable bacteria studied to date. Notably, type IV pili are encoded by both Gram-negative and Gram-positive bacterial species. The formation of Gram-negative type IV pili is modeled in Fig. 3.1(c), and the differences with Gram-positive type IV pili are briefly discussed in the figure legend, type IV pili serve a variety of roles among different species. Unlike CUP and sortase-dependent pili that have not yet been shown to possess the ability to disassemble, type IV pili are able to assemble and disassemble (Merz et al., 2000). In Gram-negative bacteria, type IV pilus biogenesis occurs through a machinery that spans both the membranes and requires ATP for energy production (Fig. 3.1(c); Fronzes et al., 2008). However, the double-membrane spanning machinery supporting the extrusion of the type IV pili can vary depending on the bacterial species (Ayers et al., 2010).
Type IV pili are also known as bundle-forming pili because bacteria such as V cholerae and enteropathogenic E. coli (EPEC) can express these pili in bundles to promote twitching motility, aggregation, and biofilm formation (Burrows, 2005; Craig et al., 2003; Cleary et al., 2004). Clostridium species are reported to utilize type IV pili for gliding across surfaces, therefore mediating primary engagement with a surface, as discussed in Section 3.2.2 (Varga et al., 2006). Mycobacterium species use type IV pili to adhere to host cell and abiotic surfaces (Ramsugit and Pillay, 2015).