Type IV pili (T4P) are widespread and highly dynamic hair-like bacterial appendages mediating a wide range of functions: DNA uptake, motility, adhesion etc. The bona fide T4P is mainly composed of major pilins but also contains minor pilins, whose functions are likely to be diverse but remain poorly defined. How T4P mediate adhesion is not understood as in other well characterised adhesive pili where tip-located adhesin subunits mediate direct binding to human cells and/or structures. In my thesis, I have addressed this question using the Gram-positive opportunistic pathogen Streptococcus sanguinis, which assembles retractable T4P using a simpler machinery. Importantly, S. sanguinis filaments have two unusual features: they are composed of two major pilins in a 4/3 ratio (PilE1 and PilE2), and they contain three additional pilin-like proteins with a conserved N-terminal pilin domain (PilA, PilB and PilC). The last two have specific additional domains grafted at their C-terminus. My work has shown that PilB, suggested to be tip-localised, exhibits an unusual bimodular pilin structure with a
bulky vWA module which shows preferential binding to host components. PilB VWA domain also harbors a canonical MIDAS, which preferential binding to Mg2+ and Mn2+ is involved in twitching motility and binding to eukaryotic cells. I also found that PilC utilises a graft ConA-like domain which most likely recognises sialylated glycans. In parallel, I was also able to observe S. sanguinis T4P dynamics in real-time, demonstrating that it is similar to Gram-negative species. By identifying a new wide class of modular pilins and providing a detailed structure/function of one of them, this study significantly improves the general understanding of the molecular mechanisms of T4P-mediated adhesion.