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Using site-directed mutagenesis in coijunction with NMR structural data on the adhesion domain of human CD2, we have defmed the binding region for CD58. Previous structural studies of rat and human CD2 indicate that this adhesion domain is immunoglobulin-like. Here we report that the CD58 binding site is a well-circumscribed, charged surface area covering -770 A2 on the AGFCC'C" face of the CD2 (8 barrel. This site contains 13-strand residues in the carboxyl-terminal half of the F strand (including Lys-82 and Tyr-86), the top of the C strand , and the C' strand (Gln-46), which are all solvent exposed. In addition, several exposed residues on the FG loop , the CC' loop (Lys-41 and Lys-43), and the C'C" loop (Arg-48 and Lys-51) form this site. In contrast, neither residues on the more peripheral G and C" strands of the same CD2 surface nor residues on B, E, and D strands of the opposite face are involved in CD58 binding. This CD58 binding site is predicted to lie most distal to the T-lymphocyte surface membrane, with ready access to CD58 on the surface of the opposing antigen-presenting cell.The interactions between helper T lymphocytes and their cognate partners (including antigen-presenting cells, epithelial cells, and endothelial cells) and between cytolytic T lymphocytes and target cells are predominantly mediated by adhesion molecules (reviewed in refs. 1 and 2). The transmembrane T-cell glycoprotein CD2 is an important component of these processes, functioning to promote the initial stages of cell contact even prior to recognition of antigen and major histocompatibility complex (MHC) by the T-cell receptor (TCR) (3, 4). CD2 is expressed on all T lymphocytes and the vast majority of thymocytes (5). Human CD2 binds to the cell surface glycoprotein CD58 (LFA-3), which is ubiquitously expressed on many cell types including antigenpresenting cells, thereby facilitating cell-cell adhesion (6, 7). The extracellular segment of CD2 comprises two domains with the membrane distal N-terminal domain mediating its adhesion function (8,9). Although the monomeric affinity of CD2 for CD58 is only micromolar, rapid CD58-dependent reorganization of CD2 at the interface between the T cell and the antigen-presenting cell readily establishes intercellular interactions via multipoint attachment (4, 10). Not surprisingly, antibodies directed against either CD2 or CD58 can block human T-cell activation (7).Recently, a detailed structure of the glycosylated adhesion domain ofhuman CD2 has been obtained by NMR and shows a typical immunoglobulin fold (11,12). This study complements and extends earlier studies on the structure of rat CD2The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact. (13,14). However, the human CD2 molecule differs from its rodent homologue in several major respects. First, the physiologic ligand of human CD2 is CD58, whereas the rat CD2 ligand is CD48 ...

The goal of this work is to define the structural and sequence features common to sandwich-like proteins (SPs), a group of very different proteins now comprising 69 superfamilies in 38 protein folds. Analysis of the arrangements of strands within main sandwich sheets revealed a rigorously defined constraint on the supersecondary substructure that holds true for 94% of known SP structures. The invariant substructure consists of two interlocked pairs of neighboring ␤-strands. It is even more typical for centers of SP than the well-known ''Greek key'' strands arrangement for their edges. As homology among these proteins is not usually detectable even with the most powerful sequence-comparing algorithms, we employed a structure-based approach to sequence alignment. Within the interlocked strands we found 12 positions with fixed structural roles in SP. A residue at any of these positions possesses similar structural properties with residues in the same position of other SPs. The 12 positions lie at the center of the interface between the ␤-sheets and form the common geometrical core of SPs. Of the 12 positions, 8 are occupied by only four hydrophobic residues in 80% of all SPs.