The three-dimensional structure of the hy-

Multiple binding capabilities utilized by specific protein-to-protein interactions in molecular recognition events are being documented increasingly but remain poorly understood at the molecular level. We identified five unrelated peptides that compete with each other for binding to the paratope region of the monoclonal anti-p24 (HIV-1) antibody CB4-1 by using a synthetic positional scanning combinatorial library XXXX[B1,B2,B3,X1,X2,X3]XXXX (14 mers; 68,590 peptide mixtures in total) prepared by spot synthesis. Complete sets of substitution analogs of the five peptides revealed key interacting residues, information that led to the construction of binding supertopes derived from each peptide. These supertope sequences were identified in hundreds of heterologous proteins, and those proteins that could be obtained were shown to bind CB4-1. Implications of these findings for immune escape mechanisms and autoimmunity are discussed.

specific, the phenomena cross-reactivity and polyspecificity are observed quite frequently (see Kramer et al., 1997 [this issue of Cell]). As described previously, we suggest discriminating between the terms crossreactivity and polyspecificity; the term cross-reactivity

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The hybrid Bacillus (1,3–1,4)‐β‐glucanase H(A16‐M), consisting of 16 N‐terminal amino acids derived from the mature form of the B. amyloliquefaciens enzyme and of 198 C‐proximal amino acids from the B. macerans enzyme, binds a calcium ion at a site at its molecular surface remote from the active center [T. Keitel, O. Simon, R. Borriss & U. Heinemann (1993) Proc. Natl Acad. Sci. USA 90, 5287–5291]. X‐ray diffraction analysis at 0.22‐nm resolution of crystals grown in the absence of calcium and in the presence of EDTA shows this site to be occupied by a sodium ion. Whereas the calcium ion has six oxygen atoms in its coordination sphere, two of which are from water molecules, sodium is fivefold coordinated with a fifth ligand belonging to a symmetry‐related protein molecule in the crystal lattice. The affinity of H(A16‐M) for calcium over sodium has been determined calorimetrically. Calcium binding stabilizes the native three‐dimensional structure of the protein as shown by guanidinium chloride unfolding and thermal inactivation experiments. The enhanced enzymic activity of Bacillusβ‐glucanases at elevated temperatures in the presence of calcium ions is attributed to a general stabilizing effect by the cation.

H(A16-M) is a hybrid endo-1,3-1,4-beta-D-glucan 4-glucanohydrolase from Bacillus. Its crystal structure was refined using synchrotron X-ray diffraction data up to a maximal resolution of 0.16 nm. The R value of the resulting model is 14.3% against 21,032 reflections > 2 sigma. 93% of the amino acid residues are in the most favorable regions of the Ramachandran diagram, and geometrical parameters are in accordance with other proteins solved at high resolution. As shown earlier [Keitel, T., Simon, O., Borriss, R. & Heinemann, U. (1993) Proc. Natl Acad. Sci. USA 90, 5287-5291], the protein folds into a compact jellyroll-type beta-sheet structure. A systematic analysis of the secondary structure reveals the presence of two major antiparallel beta-sheets and a three-stranded minor mixed sheet. Amino acid residues involved in catalysis and substrate binding are located inside a deep channel spanning the surface of the protein. To investigate the stereochemical cause of the observed specificity of endo-1,3-1,4-beta-D-glucan 4-glucanohydrolases towards beta-1,4 glycosyl bonds adjacent to beta-1,3 bonds, the high-resolution crystal structure has been used to model an enzyme-substrate complex. It is proposed that productive substrate binding to the subsites p1, p2 and p3 of H(A16-M) requires a beta-1,3 linkage between glucose units bound to p1 and p2.

H(A16‐M) is a hybrid endo‐1,3‐1,4‐β‐D‐glucan 4‐glucanohydrolase from Bacillus. Its crystal structure was refined using synchrotron X‐ray diffraction data up to a maximal resolution of 0.16 nm. The R value of the resulting model is 14.3% against 21032 reflections >2σ. 93% of the amino acid residues are in the most favorable regions of the Ramachandran diagram, and geometrical parameters are in accordance with other proteins solved at high resolution. As shown earlier [Keitel, T., Simon, O., Borriss, R. & Heinemann, U. (1993) Proc. Natl Acad. Sci. USA 90, 5287–5291], the protein folds into a compact jellyroll‐type β‐sheet structure. A systematic analysis of the secondary structure reveals the presence of two major antiparallel β‐sheets and a three‐stranded minor mixed sheet. Amino acid residues involved in catalysis and substrate binding are located inside a deep channel spanning the surface of the protein. To investigate the stereochemical cause of the observed specificity of endo‐1,3‐1,4‐β‐D‐glucan 4‐glucanohy‐drolases towards β‐1,4 glycosyl bonds adjacent to β‐1,3 bonds, the high‐resolution crystal structure has been used to model an enzyme‐substrate complex. It is proposed that productive substrate binding to the subsites p1, p2 and p3 of H(A16‐M) requires a β‐1,3 linkage between glucose units bound to p1 and p2.

H(A16‐M) is a hybrid endo‐1,3‐1,4‐β‐D‐glucan 4‐glucanohydrolase from Bacillus. Its crystal structure was refined using synchrotron X‐ray diffraction data up to a maximal resolution of 0.16 nm. The R value of the resulting model is 14.3% against 21032 reflections >2σ. 93% of the amino acid residues are in the most favorable regions of the Ramachandran diagram, and geometrical parameters are in accordance with other proteins solved at high resolution. As shown earlier [Keitel, T., Simon, O., Borriss, R. & Heinemann, U. (1993) Proc. Natl Acad. Sci. USA 90, 5287–5291], the protein folds into a compact jellyroll‐type β‐sheet structure. A systematic analysis of the secondary structure reveals the presence of two major antiparallel β‐sheets and a three‐stranded minor mixed sheet. Amino acid residues involved in catalysis and substrate binding are located inside a deep channel spanning the surface of the protein. To investigate the stereochemical cause of the observed specificity of endo‐1,3‐1,4‐β‐D‐glucan 4‐glucanohy‐drolases towards β‐1,4 glycosyl bonds adjacent to β‐1,3 bonds, the high‐resolution crystal structure has been used to model an enzyme‐substrate complex. It is proposed that productive substrate binding to the subsites p1, p2 and p3 of H(A16‐M) requires a β‐1,3 linkage between glucose units bound to p1 and p2.