The Dynamic Structures of the Type IV Pilus

Abstract

Type IV pilus (T4P)-like systems have been identified in almost every major phylum of prokaryotic life. They include the type IVa pilus (T4aP), type II secretion system (T2SS), type IVb pilus (T4bP), Tad/Flp pilus, Com pilus, and archaeal flagellum (archaellum). These systems are used for adhesion, natural competence, phage adsorption, folded-protein secretion, surface sensing, swimming motility, and twitching motility. The T4aP allows for all of these functions except swimming and is therefore a good model system for understanding T4P-like systems. Recent structural analyses have revolutionized our understanding of how the T4aP machinery assembles and functions. Here we review the structure and function of the T4aP.

FIGURE 1

Subcomplexes of the T4aP. The protein structures portrayed reflect the full-length structure predictions and their predicted location in the T4aP. This figure is largely consistent with the previously published working model of the M. xanthus T4aP (43). Due to limited information, there is uncertainty regarding the locations of PilF, TsaP, PilY1, and the minor pilins.

FIGURE 2

The structures of the type IV pilus. The four subcomplexes are split into four quadrants, which are further subdivided into individual proteins in boxes colored to correspond to Fig. 1. In the linear domain architecture, domains are displayed to scale as blocks colored to indicate known structures (rainbow colors), segments with high-confidence structure predictions (gray), unknown structures (white), transmembrane segments (diagonal bars), hydrolyzed signal peptides (black), or predicted/known disorder (black line). The known or predicted domain name is written; if a domain has no name, it could not be predicted. In the black outlined cartoon structures, a black outline of the predicted (–129) full-length homology model is shown to scale for reference. Known structures are displayed as cartoons in rainbow colors corresponding to the colors shown in the linear domain architecture. A short description of the rainbow-colored cartoon structure and the PDB accession code are written in black font. Since the black outline is a P. aeruginosa structure prediction while the cartoons correspond to structures sometimes determined in other species, the black outline and cartoons may not fully match. Note that the PDB coordinate file for PilQ from T. thermophilus (marked with an asterisk) was obtained from the authors of reference and used here with their permission; only the secretin and adjacent N1 domain (N5 in T. thermophilus) are shown here, as the other T. thermophilus PilQ domains are divergent or atypical compared to those in P. aeruginosa. (The N1 domain is also named N2, N3, N4, or N5 in systems or species where the N1 domain is duplicated.) No black outline is shown for FimV, as high-confidence structure prediction was not possible for most of this component. Unexpectedly, most of TsaP was predicted (127) with high confidence to be structurally similar to the protein with PDB code 3SLU. Gray boxes note interesting features of the protein or other relevant structures; structures in gray boxes are not to scale.