A Distinct Type of Pilus from the Human Microbiome

Abstract

Summary Pili are proteinaceous polymers of linked pilins that protrude from the cell surface of many bacteria and often mediate adherence and virulence. We investigated a set of 20 Bacteroidia pilins from the human microbiome whose structures and mechanism of assembly were unknown. Crystal structures and biochemical data revealed a diverse protein superfamily with a common Greek-key β-sandwich fold with two transthyretin-like repeats that polymerize into a pilus through a strand-exchange mechanism. The assembly mechanism of the central, structural pilins involves proteinase-assisted removal of their N-terminal β-strand, creating an extended hydrophobic groove that binds the C-terminal donor strands of the incoming pilin. Accessory pilins at the tip and base have unique structural features specific to their location, allowing initiation or termination of the assembly. The bacteroidia pilus therefore has a biogenesis mechanism that is distinct from other known pili and likely represents a different type of bacterial pilus.

A comprehensive guide to pilus biogenesis in Gram-negative bacteria

“…, and thus serve to cap the pilus structure 116,122 . Anchor pilins at the pilus base are also unique because they are not processed by Rgp or Kgp and retain their N-terminal lipid modification, thereby anchoring the pilus into the OM 116 (FIG.…”

“…Interestingly, lectin or lectin-like domains are versatile carbohydrate-interacting domains found in many pilus types. Both CU and type V pili contain distal 'tip' or 'adhesin' subunits; these proteins are fusions of pilin and lectin domains that allow their successful incorporation into pili and confer adhesive properties, respectively 14,15,116 . One of the most fascinating properties of certain pili is their ability to 'stretch'.…”

“…In the open conformation, a hydrophobic groove is exposed along the length of the CTD where the A1' strand would normally extend the β-sheet. Moreover, after the Rgp or Kgp-assisted removal of the A1 β-strand of the NTD at the OM, and with the A1' and A2' strands of the CTD in an open conformation, a hydrophobic groove is exposed that runs along the entire protein (NTD and CTD) 116 (FIG. 4c).…”

“…Both the N-and C-terminal regions have been proposed to serve as the donor strand during this process 116,122 . However, cysteine crosslinking experiments support the idea that the next subunit's flexible Cterminal A1 'and A2' strands are involved in donor strand exchange 116 . Tip pilins that have been structurally characterised so far either lack the flexible A1' and A2' strands (e.g.…”

“…It has been proposed that a strand-exchange mechanism whereby the hydrophobic groove is complemented by a strand from the neighbouring subunit allows pilus assembly 116,122 . Both the N-and C-terminal regions have been proposed to serve as the donor strand during this process 116,122 . However, cysteine crosslinking experiments support the idea that the next subunit's flexible Cterminal A1 'and A2' strands are involved in donor strand exchange 116 .…”