The Camelidae is the only taxonomic family known to possess functional heavy-chain antibodies, lacking light chains. We report here the 2.5 A resolution crystal structure of a camel VH in complex with its antigen, lysozyme. Compared to human and mouse VH domains, there are no major backbone rearrangements in the VH framework. However, the architecture of the region of VH that interacts with a VL in a conventional FV is different from any previously seen. Moreover, the CDR1 region, although in sequence homologous to human CDR1, deviates fundamentally from the canonical structure. Additionally, one half of the CDR3 contacts the VH region which in conventional immunoglobulins interacts with a VL whereas the other half protrudes from the antigen binding site and penetrates deeply into the active site of lysozyme.

The interactions of RNase A with cytidine 3'-monophosphate (3"CMP) and deoxycytidyl-3',5'-deoxyadenosine (d(CpA)) were analyzed by X-ray crystallography. The 3'-CMP complex and the native structure were determined from trigonal crystals, and the d(CpA) complex from monoclinic crystals. The differences between the overall structures are concentrated in loop regions and are relatively small. The protein-inhibitor contacts are interpreted in terms of the catalytic mechanism. The general base His 12 interacts with the 2' oxygen, as does the electrostatic catalyst Lys 41. The general acid His 119 has 2 conformations (A and B) in the native structure and is found in, respectively, the A and the B conformation in the d(CpA) and the 3'-CMP complex. From the present structures and from a comparison with RNase T1, we propose that His 119 is active in the A conformation. The structure of the d(CpA) complex permits a detailed analysis of the downstream binding site, which includes His 119 and Asn 71. The comparison of the present RNase A structures with an inhibitor complex of RNase T1 shows that there are important similarities in the active sites of these 2 enzymes, despite the absence of any sequence homology. The water molecules were analyzed in order to identify conserved water sites. Seventeen water sites were found to be conserved in RNase A structures from 5 different space groups. It is proposed that 7 of those water molecules play a role in the binding of the N-terminal helix to the rest of the protein and in the stabilization of the active site.

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The structure of phytohemagglutinin-L (PHA-L), a leucoagglutinating seed lectin from Phaseolus vulgaris, has been solved with molecular replacement using the coordinates of lentil lectin as model, and refined at a resolution of 2.8 Å. The final R-factor of the structure is 20.0%. The quaternary structure of the PHA-L tetramer differs from the structures of the concanavalin A and peanut lectin tetramers, but resembles the structure of the soybean agglutinin tetramer. PHA-L consists of two canonical legume lectin dimers that pack together through the formation of a close contact between two ␤-strands. Of the two covalently bound oligosaccharides per monomer, only one GlcNAc residue per monomer is visible in the electron density. In this article we describe the structure of PHA-L, and we discuss the putative position of the high affinity adenine-binding site present in a number of legume lectins. A comparison with transthyretin, a protein that shows a remarkable resemblance to PHA-L, gives further ground to our proposal.

The structure of concanavalin A in complex with the trimannoside methyl-3,6-di-O-(␣-D-mannopyranosyl)-␣-Dmannopyranoside has been determined in a novel space group. In three of the four subunits of the concanavalin A tetramer, the interactions between the protein and the bound saccharide are essentially identical to those reported previously by other authors (Naismith, J. H., and Field, R. A. (1996) J. Biol. Chem. 271, 972-976). In the fourth subunit, however, the ␣133 linkage has a different conformation, resulting in a different part of the ␣133-linked mannose interacting with essentially the same surface of the protein. Furthermore, significant differences are observed in the quaternary associations of the subunits compared with the saccharide-free structures and other carbohydrate complexes, suggesting that the concanavalin A tetramer is a rather flexible entity.Concanavalin A was the first legume lectin to be isolated (1), sequenced (2-4), and to have its three-dimensional structure determined by x-ray crystallography (5, 6). Concanavalin A belongs to the glucose/mannose-specific lectins (7) and was shown to have its highest binding affinity for methyl-3, Nevertheless, the first view of a carbohydrate bound to concanavalin A in a specific manner was obtained only in 1989, when the structure of a concanavalin A-methyl-␣-D-mannopyranoside complex was presented (12). Today, the details of the trimannose 1 binding to concanavalin A have been unravelled by Naismith and Field (13) in a crystal form with a unit cell different from the structure discussed by us. The structure of concanavalin A complexed with methyl-␣-D-glucopyranoside in a cubic crystal has been solved (14) and co-crystallization with methyl-␣-D-arabinofuranoside (15) has been reported. Also, the structures of a number of other legume lectins and their carbohydrate complexes have been determined (16 -19). On the other hand the thermodynamics of concanavalin A-carbohydrate interactions have been addressed in several recent studies and have been the subject of considerable debate (20 -23). A refined structure of concanavalin A in complex with the specific trisaccharide methyl-3,6-di-O-(␣-D-mannopyranosyl)-␣-D-mannopyranoside is here compared with the recently published structure of Naismith and Field (13). EXPERIMENTAL PROCEDURES Purification of concanavalin A, crystallization of its complex with the methyl-3,6-di-O-(␣-D-mannopyranosyl)-␣-D-mannopyranoside, cryocooling and data collection have been reported elsewhere (24). Crystals belong to space group P2 1 , unit cell parameters a ϭ 59.83 Å, b ϭ 62.84 Å, c ϭ 125.92 Å, ␤ ϭ 93.87°and contain a complete concanavalin A tetramer in the asymmetric unit. Data within 15.0 and 2.3 Å were collected at 100 K from a single crystal with a FAST area detector on an Enraf-Nonius rotating anode source operated at 40 kV and 90 mA. Due to technical problems, the data collection had to be abandoned prematurely, resulting in a data set that is 71.3% complete between the resolution limits of 10 and 2.35 Å. Further attem...