Structure/function analysis of a complex of minor pilins in the Streptococcus sanguinis Type IV Pili, mediating binding to host glycans

Abstract

Type IV pili (T4P) are widespread multi-functional prokaryotic filaments. Typically composed of one major pilin, T4P also contain minor pilins present at a lower abundance. Minor pilins contribute to the assembly and functional versatility of the filaments but their precise role in T4P biology remains poorly understood. The Gram-positive opportunistic pathogen Streptococcus sanguinis provides a simple T4P machinery for studying the involvement of minor pilins in pili biogenesis. Its retractile pili comprise two major pilins and three minor pilins – PilA, PilB and PilC. While previous work has described the two major pilins and the minor pilin PilB, this study characterises PilA and PilC. PilA is a small minor pilin structurally homologous to the T2SS pseudopilin GspI. PilC is a trimodular minor pilin with a functional C-terminal lectin domain. We show that PilA and PilC form a heterodimeric complex supported by an intermolecular ß-sheet between PilA and the pilin domain of PilC. The interaction allows PilA to act as a chaperone for PilC, stabilising the PilC pilin domain by ß-strand complementation. Modelling suggests that the PilA-PilC complex resides at the tip of the filaments. PilA is incorporated first, followed by PilC whose large C-terminal domains cap the pili. The PilA-PilC complex thus facilitates the presentation of the PilC lectin module by the filaments. The lectin module binds to the terminal moieties of sialylated glycans with moderate affinity. The interaction is mediated by a binding pocket on the concave side of the lectin ß-sandwich fold. The role that PilC plays in the S. sanguinis life cycle, however, remains to be established. Altogether, this study provides the first description of a minor pilin complex in Gram-positive T4P and completes the first characterisation of the role of each pilus subunit in a T4F system.