Ace is a collagen-binding MSCRAMM from Enterococcus faecalis

A putative collagen-binding MSCRAMM, Ace, of Enterococcus faecalis was iden tified by searching bacterial genome data bases for proteins containing dom ains homologous to the ligand-binding region of Cna, the collagen-binding M SCRAMM from Staphylococcus aureus. Ace was predicted to have a molecular ma ss of 71 kDa and contains features characteristic of cell surface proteins on Gram-positive bacteria, including a LPXTG motif for cross-linking to the cell wall. The N-terminal region of Ace contained a region (residues 174-3 19) in which 56% of the residues are identical or similar when compared wit h the minimal ligand-binding region of Cna (Cna 151-318); the remainder of the Ace A domain has 46% similarity with the corresponding region of the Cn a A domain. Antibodies raised against recombinant Ace A domain were used to verify the cell surface expression of Ace on E. faecalis. These antibodies also effectively inhibited the adhesion of enterococcal cells to a collage n substrate, suggesting that Ace is a functional collagen-binding MSCRAMM. Structural modeling of the conserved region in Ace (residues 174-319) sugge sted a structure very similar to that reported for residues 151-318 of the Cna collagen-binding domain in which the ligand-binding site was identified as a trench transversing a beta-sheet face (Symersky, J., Patti, J. M., Ca rson, M., House-Pompeo, K., Teale, M., Moore, D., Jin, L., DeLucas, L. J., Hook, M., and Narayana, S. V. L. (1997) Nat. Struct. Biol. 10, 833-838). Bi ochemical analyses of recombinant Ace and Cna A domains supported the model ing data in that the secondary structures were similar as determined by CD spectroscopy and both proteins bound at multiple sites in type I collagen w ith micromolar affinities, but with different apparent kinetics. We conclud e that Ace is a collagen-binding MSCRAMM on enterococci and is structurally and functionally related to the staphylococcal Cna protein.