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

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 S. sanguinis displays an unusual bimodular 3D structure, with a bulky von Willebrand factor A-like (vWA) module "grafted" onto a small pilin module via a short unstructured loop. Structural modelling suggests that PilB is only compatible with a localisation at the tip of T4P. By performing a detailed functional analysis, we found that (i) the vWA module contains a canonical metal ion-dependent adhesion site (MIDAS), preferentially binding Mg2+ and Mn2+, (ii) abolishing metal-binding has no impact on the structure of PilB or piliation, (iii) metal-binding is important for S. sanguinis T4P-mediated twitching motility and adhesion to eukaryotic cells, and (iv) 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 an adhesive 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.