Roles for Glycosylation of Cell Surface Receptors Involved in Cellular Immune Recognition Pauline M. Rudd 1 *, Mark R. Wormald 1 , Robyn L. Stanfield 2 , Mingdong Huang 2 , Niklas Mattsson 2 , Jeffrey A. Speir 2 , Jeannine A. DiGennaro 2 , Jacquelyn S. Fetrow 2 , Raymond A. Dwek 1 and Ian A. Wilson 2 * 1 Department of Biochemistry University of Oxford, The Glycobiology Institute, South Parks Road, Oxford OX1 3QU UK 2 Department of Molecular Biology and Skaggs Institute for Chemical Biology, The Scripps Research Institute 10550 No. Torrey Pines Road La Jolla, CA 92037, USA The majority of cell surface receptors involved in antigen recognition by T cells and in the orchestration of the subsequent cell signalling events are glycoproteins. The length of a typical N-linked sugar is comparable with that of an immunoglobulin domain (30 A Ê ). Thus, by virtue of their size alone, oligosaccharides may be expected to play a signi®cant role in the functions and properties of the cell surface proteins to which they are attached. A databank of oligosaccharide structures has been constructed from NMR and crystallographic data to aid in the interpretation of crys- tal structures of glycoproteins. As unambiguous electron density can usually only be assigned to the glycan cores, the remainder of the sugar is then modelled into the crystal lattice by superimposing the appropriate oligosaccharide from the database. This approach provides insights into the roles that glycosylation might play in cell surface receptors, by pro- viding models that delineate potential close packing interactions on the cell surface. It has been proposed that the speci®c recognition of antigen by T cells results in the formation of an immunological synapse between the T cell and the antigen-presenting cell. The cell adhesion glycoproteins, such as CD2 and CD48, help to form a cell junction, providing a molecu- lar spacer between opposing cells. The oligosaccharides located on the membrane proximal domains of CD2 and CD48 provide a scaffold to ori- ent the binding faces, which leads to increased af®nity. In the next step, recruitment of the peptide major histocompatibility complex (pMHC) by the T-cell receptors (TCRs) requires mobility on the membrane surface. The TCR sugars are located such that they could prevent non-speci®c aggregation. Importantly, the sugars limit the possible geometry and spa- cing of TCR/MHC clusters which precede cell signalling. We postulate that, in the ®nal stage, the sugars could play a general role in controlling the assembly and stabilisation of the complexes in the synapse and in protecting them from proteolysis during prolonged T-cell engagement. # 1999 Academic Press Keywords: protein glycosylation; cell surface receptors; TCR-MHC interactions; carbohydrate structure; signal transduction *Corresponding authors E-mail addresses of the corresponding authors: pmr@glycob.ox.ac.uk; wilson@scripps.edu Present address: J. A. DiGennaro and J. S. Fetrow, GeneFormatics, Inc., 5830 Oberlin Drive, Suite 200, San Diego, CA 92121, USA. Abbreviations used: APC, antigen-presenting cell; MHC, major histocompatibility complex; NMR, nuclear magnetic resonance; PDB, protein data bank; pMHC, peptide-MHC; TCR, T-cell receptor; A2, biantennary complex type sugar; G, galactose; F, fucose; GlcNAc, N-acetylglucosamine; S, sialic acid; M, mannose; B, bisecting GlcNAc; GPI, glycosyl phosphatidyl inositol; Ig, immunoglobulin; DAF, decay accelerating factor; CCP, complement control proteins. Article No. jmbi.1999.3104 available online at http://www.idealibrary.com on J. Mol. Biol. (1999) 293, 351±366 0022-2836/99/420351±16 $30.00/0 # 1999 Academic Press