Crystallization and X-ray Crystallographic Analysis of the Adhesive SpaC Pilin Subunit in the SpaCBA Pilus of Gut-adapted Lactobacillus rhamnosus GG.

Gram-positive Lactobacillus rhamnosus GG, a gut-adapted commensalic (and probiotic) strain, is known to express sortase-assembled pili on its cell surface. These SpaCBA-called pili consist of three different types of building blocks; the SpaA backbone-pilin subunit and the SpaB and SpaC ancillary pilins. SpaC is a relatively large (~90kDa) multi-domain fimbrial adhesin, and while it is located primarily at the SpaCBA pilus tip, occasionally, it can also be detected throughout the length of pilus backbone. Functionally, SpaC mainly accounts for SpaCBA pilus-mediated interactions with intestinal mucus, collagen, and human gut epithelial cells. Moreover, SpaC adhesiveness is also perceived to have a causal relationship with SpaCBA pilus-induced host-cell immune responses. In order to improve the mechanistic understanding of SpaC and its adhesive properties by structural investigation, we purified and successfully crystallized a recombinant construct of the near full-length SpaC protein (residues 36-856) in the presence of magnesium ions. X-ray diffraction data were collected to 2.6 A resolution. The SpaC crystal belongs to the space group P2 1 2 1 2 with unit cell parameters a = 116.5, b = 128.3, c = 136.5 A and contains two molecules in the asymmetric unit. Presence of conserved metal ion-dependent adhesion site containing von Willebrand factor type A domain suggests its likely role in the function of SpaC.