Summary auto-generated
This study identifies three new genes (sdrC, sdrD, and sdrE) in Staphylococcus aureus strain Newman that encode serine-aspartate (SD) repeat-containing proteins structurally related to the fibrinogen-binding clumping factors ClfA and ClfB. These genes are closely linked and tandemly arranged. The Sdr proteins share similar domain organization with Clf proteins, including an N-terminal signal sequence, an approximately 500-residue A region, and C-terminal cell wall-anchoring domains. The key distinction is that Sdr proteins contain 2-5 additional B-motifs (110-113 residues each) between regions A and R, with each B-motif containing a calcium-binding EF-hand loop. Recombinant SdrD protein analysis using bisANS fluorescence demonstrated that structural integrity depends on calcium binding; the protein adopts an unfolded conformation without calcium but refolds upon calcium addition. Southern hybridization analysis of 31 S. aureus strains from diverse human and bovine sources revealed the sdr locus is widespread, though some strains contained two rather than three genes.
Key findings
- Three new genes (sdrC, sdrD, sdrE) encode proteins with serine-aspartate repeats and calcium-binding EF-hand motifs in tandemly arranged B-repeats
- SdrD protein structure is calcium-dependent, unfolding without Ca2+ and restoring native conformation upon calcium binding
- The conserved motif TYTFTDYVD appears in all five members of the SD repeat protein family, suggesting common ancestry
- The sdr locus is present in all 31 tested S. aureus strains from human and bovine sources, indicating biological importance
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Abstract
Three new genes encoding the serine-aspartate (SD) repeat-containing proteins SdrC, SdrD and SdrE were found in Staphylococcus aureus strain Newman. The SD repeats had earlier been found in the S. aureus fibrinogen-binding clumping factors ClfA and ClfB. The clfA and clfB genes encode high-molecular-mass fibrinogen-binding proteins that are anchored to the cell surface of S. aureus. The sdr genes now reported are closely linked and tandemly arrayed. The putative Sdr proteins have both organizational and sequence similarity to ClfA and ClfB. At the N- terminus, putative secretory signal sequences precede approximately 500 residue A regions. The A regions of the Sdr and Clf proteins exhibit only 20-30% residue identity when aligned with any other member of the family. The only conserved sequence is the consensus motif TYTFTDYVD. The Sdr proteins differ from ClfA and ClfB by having two to five additional 110-113 residue repeated sequences (B-motifs) located between region A and the R-region. Each B-motif contains a consensus Ca2+-binding EF-hand loop normally found in eukaryotic proteins. The structural integrity of recombinant SdrD(B1-B5) protein comprising the five B-repeats of SdrD was shown by bisANS fluorescence analysis to be Ca2+-dependent, suggesting that the EF-hands are functional. When Ca2+ was removed the structure collapsed to an unfolded conformation. The original structure was restored by addition of Ca2+. The C-terminal R- domains of the Sdr proteins contain 132-170 SD residues. These are followed by conserved wall-anchoring regions characteristic of many surface proteins of Gram-positive bacteria. The sdr locus was present in all 31 S. aureus strains from human and bovine sources tested by Southern hybridization, although in a few strains it contained two rather than three genes.