SdrC induces staphylococcal biofilm formation through a homophilic interaction

Abstract

Summary The molecular pathogenesis of many Staphylococcus aureus infections involves growth of bacteria as biofilm. In addition to polysaccharide intercellular adhesin (PIA) and extracellular DNA, surface proteins appear to mediate the transition of bacteria from planktonic growth to sessile lifestyle as well as biofilm growth, and can enable these processes even in the absence of PIA expression. However, the molecular mechanisms by which surface proteins contribute to biofilm formation are incompletely understood. Here we demonstrate that self-association of the serine-aspartate repeat protein SdrC promotes both bacterial adherence to surfaces and biofilm formation. However, this homophilic interaction is not required for the attachment of bacteria to abiotic surfaces. We identified the subdomain that mediates SdrC dimerization and subsequent cell-cell interactions. In addition, we determined that two adjacently located amino acid sequences within this subdomain are required for the SdrC homophilic interaction. Comparative amino acid sequence analysis indicated that these binding sites are conserved. In summary, our study identifies SdrC as a novel molecular determinant in staphylococcal biofilm formation and describes the mechanism responsible for intercellular interactions. Furthermore, these findings contribute to a growing body of evidence suggesting that homophilic interactions between surface proteins present on neighboring bacteria induce biofilm growth.

Molecular interactions and inhibition of the staphylococcal biofilm-forming protein SdrC

“…1B). These data show that SdrC induces intercellular adhesion (24), and that the protein is properly exposed on the cell surface.…”

“…Neither the number of domains nor their lengths match the structure of SdrC, which is made of five larger domains, namely three N subdomains comprising 132, 153, and 162 residues and two B domains of ∼104 residues. The 25-nm-length increment may result from the unfolding of the homophilic binding region of the N2 subdomain [amino acid sequences RPGSV [247][248][249][250][251] and VDQYT 288-292 (24)]. We propose that the periodic force peaks reflect the rupture of multiple homophilic interactions one by one, in a zipper-like fashion.…”

“…2 E and F Cell-cell separation did not lead to the complete unfolding of the SdrC protein. Considering that each amino acid contributes 0.36 nm to the contour length of a fully extended polypeptide chain (27) and that full-length SdrC is 995 amino acids in length (24,25), the length of two fully extended proteins is expected to be 716 nm. This is much longer than the rupture lengths we observed (∼160 nm at 100 ms), thus implying that intercellular bonds rupture before complete unfolding of the proteins.…”

“…proteins and a recombinant polypeptide containing the N2 and N3 subdomains. To investigate SdrC interactions in the absence of other staphylococcal components, we focused on a Lactococcus lactis strain expressing SdrC [hereafter SdrC (+) ] (24). We also analyzed a methicillin-resistant S. aureus (MRSA) strain to support the biological relevance of our findings.…”

“…Phage display screening showed that the N2 subdomain mediates SdrC-SdrC interactions (24). Purified phage clones expressing a combination of two sequences from the N2 subdomain, RPGSV [247][248][249][250][251] and VDQYT [288][289][290][291][292] , completely inhibited the self-association (dimerization) of SdrC proteins (24). With this in mind, we measured the strength and dynamics of the SdrC homophilic interaction, using both bacterial cells expressing full-length SdrC…”