Abstract
Effective colonization of host cells by some Gram-positive bacteria often involves using lengthy, adhesive macromolecular structures called sortase-dependent pili. Among commensals, the gut-adapted Lactobacillus rhamnosus GG strain encodes the operons for two varieties of these pili (SpaCBA and SpaFED), with each structure consisting of backbone, tip, and basal pilin subunits. Although the tertiary structure was recently solved for the backbone subunit (SpaA) of the SpaCBA pilus, no structural information exists for its counterpart in the SpaFED pilus. Here, we report several crystal structures for the SpaD backbone pilin, two of which capture the N-terminal domain in either the closed (linear) or open (bent) conformation. To our knowledge, this is the first observation of the bent conformation in Gram-positive pilin structures. Based on this bent conformation, we suggest a three-stage model, which we call the expose-ligate-seal mechanism, for the docking and assembly of backbone pilins into the sortase-dependent pilus.
Highlights
Effective colonization of host cells by some Gram-positive bacteria often involves using lengthy, adhesive macromolecular structures called sortase-dependent pili
Since sortase-dependent pili are known to adhere to epithelial extracellular matrix proteins and mucus, this enables them to have an important role during host cell colonization as mechanisms of pathogenic virulence and non-pathogenic niche adaptation[6,12,13,14]
The arrival of a basal pilin attached to the housekeeping A-type sortase is the signal that initiates termination and anchoring activities[16], whereby a C-type sortase catalyzes an isopeptide bond between the basal subunit and the backbone pilin most recently added[17,18]
Summary
The full-length structure containing all three domains was later determined from orthorhombic and hexagonal crystals, which yielded two distinct conformations, respectively, called the closed and open states (Table 1). The M-domain (residues 184–358) comprises five and four-stranded β-sheets and is similar to the CnaA-type fold (Fig. 1d). In the orthorhombic crystal the three domains are arranged linearly with an elongated shape 125 Å in length. This we call GG-SpaDC for the closed conformational state. The M- and C-domain have an interface area of 567 Å2, with nine hydrogen bonds and one salt bridge. The interface area between the N- and M-domains (787 Å2) involves multiple interactions, including 14 hydrogen bonds and three salt bridges
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
