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

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.Open Acces

The Salmonella transmembrane effector SteD hijacks AP1-mediated vesicular trafficking for delivery to antigen-loading MHCII compartments.

SteD is a transmembrane effector of the Salmonella SPI-2 type III secretion system that inhibits T cell activation by reducing the amounts of at least three proteins -major histocompatibility complex II (MHCII), CD86 and CD97 -from the surface of antigen-presenting cells. SteD specifically localises at the trans-Golgi network (TGN) and MHCII compartments; however, the targeting, membrane integration and trafficking of SteD are not understood. Using systematic mutagenesis, we identify distinct regions of SteD that are required for these processes. We show that SteD integrates into membranes of the ER/Golgi through a two-step mechanism of membrane recruitment from the cytoplasm followed by integration. SteD then migrates to and accumulates within the TGN. From here it hijacks the host adaptor protein (AP)1-mediated trafficking pathway from the TGN to MHCII compartments. AP1 binding and post-TGN trafficking require a short sequence in the N-terminal cytoplasmic tail of SteD that resembles the AP1-interacting dileucine sorting signal, but in inverted orientation, suggesting convergent evolution

Structure of a heteropolymeric type 4 pilus from a monoderm bacterium

Abstract Type 4 pili (T4P) are important virulence factors, which belong to a superfamily of nanomachines ubiquitous in prokaryotes, called type 4 filaments (T4F). T4F are defined as helical polymers of type 4 pilins. Recent advances in cryo-electron microscopy (cryo-EM) led to structures of several T4F, revealing that the long N-terminal α-helix (α1) – the trademark of pilins – packs in the centre of the filaments to form a hydrophobic core. In diderm bacteria – all available bacterial T4F structures are from diderm species – a portion of α1 is melted (unfolded). Here we report that this architecture is conserved in phylogenetically distant monoderm species by determining the structure of Streptococcus sanguinis T4P. Our 3.7 Å resolution cryo-EM structure of S. sanguinis heteropolymeric T4P and the resulting full atomic model including all minor pilins highlight universal features of bacterial T4F and have widespread implications in understanding T4F biology

Characterization of a glycan-binding complex of minor pilins completes the analysis of Streptococcus sanguinis type 4 pili subunits

International audienceType 4 filaments (T4F)—of which type 4 pili (T4P) are the archetype—are a superfamily of nanomachines nearly ubiquitous in prokaryotes. T4F are polymers of one major pilin, which 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 bacterial 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 stabilizes the minor pilin PilC. We determined the structure of PilC, showing that it is a modular pilin with a lectin module binding a 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. This has general implications for T4F