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The three‐dimensional structure of the complex between the Fab fragment of an anti‐human rhinovirus neutralizing antibody (8F5) and a cross‐reactive synthetic peptide from the viral capsid protein VP2 has been determined at 2.5 A resolution by crystallographic methods. The refinement is presently at an R factor of 0.18 and the antigen‐binding site and viral peptide are well defined. The peptide antigen adopts a compact fold by two tight turns and interacts through hydrogen bonds, some with ionic character, and van der Waals contacts with antibody residues from the six hypervariable loops as well as several framework amino acids. The conformation adopted by the peptide is closely related to the corresponding region of the viral protein VP2 on the surface of human rhinovirus 1A whose three‐dimensional structure is known. Implications for the cross‐reactivity between peptides and the viral capsid are discussed. The peptide‐antibody interactions, together with the analysis of mutant viruses that escape neutralization by 8F5 suggest two different mechanisms for viral escape. The comparison between the complexed and uncomplexed antibody structures shows important conformational rearrangements, especially in the hypervariable loops of the heavy chain. Thus, it constitutes a clear example of the ‘induced fit’ molecular recognition mechanism.

The crystal structure of a family‐III cellulose‐binding domain (CBD) from the cellulosomal scaffoldin subunit of Clostridium thermocellum has been determined at 1.75 A resolution. The protein forms a nine‐stranded beta sandwich with a jelly roll topology and binds a calcium ion. conserved, surface‐exposed residues map into two defined surfaces located on opposite sides of the molecule. One of these faces is dominated by a planar linear strip of aromatic and polar residues which are proposed to interact with crystalline cellulose. The other conserved residues are contained in a shallow groove, the function of which is currently unknown, and which has not been observed previously in other families of CBDs. On the basis of modeling studies combined with comparisons of recently determined NMR structures for other CBDs, a general model for the binding of CBDs to cellulose is presented. Although the proposed binding of the CBD to cellulose is essentially a surface interaction, specific types and combinations of amino acids appear to interact selectively with glucose moieties positioned on three adjacent chains of the cellulose surface. The major interaction is characterized by the planar strip of aromatic residues, which align along one of the chains. In addition, polar amino acid residues are proposed to anchor the CBD molecule to two other adjacent chains of crystalline cellulose.