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.
Highlights
| | | | type IV pili type IV filaments type IV pilin adhesion von Willebrand factor A–like domain
We performed a global biochemical and structural analysis of S. sanguinis T4P [17], showing that 1) they are heteropolymers composed of two major pilins, PilE1 and PilE2, rather than one as usually seen, 2) the major pilins display classical type IV pilin three-dimensional (3D) structure, and 3) the filaments contain a low abundance of three minor pilins (PilA, PilB, and PilC), which are required for piliation
The larger size of PilB is due to the presence of a C-terminal domain (Fig. 1B) readily detectable by bioinformatics, which belongs to the von Willebrand factor A–like domain superfamily (InterPro entry IPR036465)
Summary
| | | | type IV pili type IV filaments type IV pilin adhesion von Willebrand factor A–like domain. The pilus subunits, type IV pilins, are characterized by a short N-terminal sequence motif known as class III signal peptide, which consists of a hydrophilic leader peptide ending with a small residue (Gly or Ala), followed by a tract of 21 predominantly hydrophobic residues [4] This tract constitutes the N-terminal segment (α-1N) of an α-helix (α-1) of ∼50 residues, which is the universally conserved structural feature in type IV pilins. Filaments are 1) assembled by a multiprotein machinery similar to diderm T4aP species but simpler with only 10 components, 2) retracted by a PilT ATPase, generating tensile forces similar to diderm species, and 3) powering intense twitching motility, leading to spreading zones around bacteria growing on plates, visible by the naked eye. The present study was prompted by a perplexing observation [i.e., the minor pilin PilB harbors a protein domain that has been
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