PilB from Streptococcus sanguinis is a bimodular type IV pilin with a direct role in adhesion

Abstract

Type IV pili (T4P) are functionally versatile filamentous nanomachines, nearly ubiquitous in prokaryotes. They are predominantly polymers of one major pilin but also contain minor pilins whose functions are often poorly defined and likely to be diverse. Here, we show that the minor pilin PilB from the T4P of Streptococcus sanguinis displays an unusual bimodular three-dimensional structure with a bulky von Willebrand factor A–like (vWA) module “grafted” onto a small pilin module via a short loop. Structural modeling suggests that PilB is only compatible with a localization at the tip of T4P. By performing a detailed functional analysis, we found that 1) the vWA module contains a canonical metal ion–dependent adhesion site, preferentially binding Mg2+ and Mn2+, 2) abolishing metal binding has no impact on the structure of PilB or piliation, 3) metal binding is important for S. sanguinis T4P–mediated twitching motility and adhesion to eukaryotic cells, and 4) the vWA module shows an intrinsic binding ability to several host proteins. These findings reveal an elegant yet simple evolutionary tinkering strategy to increase T4P functional versatility by grafting a functional module onto a pilin for presentation by the filaments. This strategy appears to have been extensively used by bacteria, in which modular pilins are widespread and exhibit an astonishing variety of architectures.

Force-Induced Changes of PilY1 Drive Surface Sensing by Pseudomonas aeruginosa

“…vWA domains have been reported in TFP-associated proteins from other organisms. For example, the vWA domain of the major pilin in Streptococcus agalactiae is essential for adhesion ( 10 ), and the MIDAS motif in the vWA domain of the major pilin in Streptococcus sanguinis has recently been shown to be important in binding to eukaryotic cells ( 11 ). Like the vWF ( 12 ), the vWA domain of the P. aeruginosa PA14 PilY1 protein also has a high number of cysteine residues; 7 out of the 11 cysteines in PilY1 are in its vWA domain.…”

The differential expression of PilY1 proteins by the HsfBA phosphorelay allows twitching motility in the absence of exopolysaccharides

“…This domain is, thus, responsible for the adhesion properties of S . sanguinis T4P [ 9 ]. It is possible that while PilY1.1 is required for binding to the exopolysaccharide fraction of the extracellular matrix, PilY1.3 binds to proteins.…”

“…Interestingly, it has been recently shown that S . sanguinis evolved a similar modularity by grafting a lectin and a von Willebrand factor A domain at the C terminus of either of two minor pilins, PilC and PilB, respectively [ 9 ]. This shows that distantly related bacterial species evolved T4P that by different strategies can adapt their adhesion ability to the environmental conditions.…”

“…T4P are also evolutionary related to Type II Secretion and archaeal flagellum systems. Binding to substrates can take place through major pilins which in some cases possess a bimodular organization [ 9 ], minor pilins such as PilV from Neisseria [ 10 ] or other pilus proteins like PilY1 from Pseudomonas aeruginosa [ 11 ].…”

Characterization of a glycan-binding complex of minor pilins completes the analysis of Streptococcus sanguinis type 4 pili subunits

“…50 residues (α1) that supports a globular head ( 4 ). However, larger minor pilins—displaying extra domains "grafted" onto a pilin moiety and hence defined as modular ( 6 )—are not uncommon, which partly explains the extreme functional versatility of T4F. Pilins are assembled in filaments at the CM ( 3 ) in two steps.…”