The crystal structure of the recombinant collagen-binding domain of Yersinia adhesin YadA from Yersinia enterocolitica serotype O:3 was solved at 1.55 Å resolution. The trimeric structure is composed of head and neck regions, and the collagen binding head region is a novel ninecoiled left-handed parallel b-roll. Before the b-roll, the polypeptide loops from one monomer to the rest, and after the b-roll the neck region does the same, making the transition from the globular head region to the narrower stalk domain. This creates an intrinsically stable 'lock nut' structure. The trimeric form of YadA is required for collagen binding, and mutagenesis of its surface residues allowed identification of a putative collagen-binding surface. Furthermore, a new structure-sequence motif for YadA b-roll was used to identify putative YadA-head-like domains in a variety of human and plant pathogens. Such domains may therefore be a common bacterial strategy for avoiding host response.

The first crystal structure of an inorganic pyrophosphatase~S-PPase! from an archaebacterium, the thermophile Sulfolobus acidocaldarius, has been solved by molecular replacement and refined to an R-factor of 19.7% at 2.7 Å. S-PPase is a D 3 homohexameric protein with one Mg 2ϩ per active site in a position similar to, but not identical with, the first activating metal in mesophilic pyrophosphatases~PPase!. In mesophilic PPases, Asp65, Asp70, and Asp102 coordinate the Mg 2ϩ , while only Asp65 and Asp102 do in S-PPase, and the Mg 2ϩ moves by 0.7 Å. S-PPase may therefore be deactivated at low temperature by mispositioning a key metal ion.The monomer S-PPase structure is very similar to that of Thermus thermophilus~T-PPase! and Escherichia colĩ E-PPase!, root-mean-square deviations around 1 Å0Ca. But the hexamer structures of S-and T-PPase are more tightly packed and more similar to each other than they are to that of E-PPase, as shown by the increase in surface area buried upon oligomerization. In T-PPase, Arg116 creates an interlocking ionic network to both twofold and threefold related monomers; S-PPase has hydrophilic interactions to threefold related monomers absent in both E-and T-PPase. In addition, the thermostable PPases have about 7% more hydrogen bonds per monomer than E-PPase, and, especially in S-PPase, additional ionic interactions anchor the C-terminus to the rest of the protein. Thermostability in PPases is thus due to subtle improvements in both monomer and oligomer interactions.