Full structure/function analysis of all the pilin subunits in a type 4 pilus: a complex of minor pilins in Streptococcus sanguinis mediates binding to glycans

Abstract

Type 4 filaments (T4F) - of which type 4 pili (T4P) are the archetype - are a superfamily of filamentous nanomachines nearly ubiquitous in prokaryotes. T4F are polymers of one major pilin that also contain minor pilins whose roles are often poorly understood. Here, we complete the structure/function analysis of the full set of T4P pilins in the opportunistic pathogen Streptococcus sanguinis. We determined the structure of the minor pilin PilA, which is unexpectedly similar to one of the subunits of a tip-located complex of four minor pilins, widely conserved in T4F. We found that PilA interacts and dramatically stabilises the minor pilin PilC. We determined the structure of PilC, showing that it is a modular pilin with a lectin module binding a specific subset of glycans prevalent in the human glycome, the host of S. sanguinis. Altogether, our findings support a model whereby the minor pilins in S. sanguinis T4P form a tip-located complex promoting adhesion to various host receptors. Our findings have general implications for a group of minor pilins widely conserved in T4F.