1H resonance assignments and secondary structure of the 13.6 kDa glycosylated adhesion domain of human CD2.

Human CD2, a glycosylated transmembrane receptor found on all T-lymphocytes, plays a key role in facilitating cellular adhesion between T-cells and target cells or antigen-presenting cells by binding to its counter receptor CD58 (LFA-3) present on the surface of those cells. All CD2 adhesion functions are localized within the amino-terminal 105-residue domain, which contains a single high mannose N-glycan required for maintaining both the conformational stability and CD58 binding properties of the glycoprotein. In order to better understand the structural basis for CD2-CD58-mediated adhesion and the critical role of the carbohydrate moiety in maintaining the functional stability of the molecule, we have determined the secondary structure of the N-glycosylated adhesion domain of human CD2 (hu-sCD2(105)) using NMR spectroscopy. Most of the 1H resonance assignments have been obtained from 1H-1H homonuclear 2D NMR spectra, which were further extended by applying 1H-15N heteronuclear 2D experiments on a hu-sCD2(105) sample selectively labeled with [15N]lysine. Thus, 98% of all backbone 1H resonances and over 80% of all side chain 1H resonances have been assigned. An overall topology characteristic of an immunoglobulin variable domain is observed, which consists of two beta-sheets comprised of three (residues 16-20, 67-71, and 60-63) and five (residues 94-103, 80-86, 32-37, 45-47, and 53-55) antiparallel beta-strands, respectively, with a hydrophobic core sandwiched between them. A ninth beta-strand (residues 7-12) makes parallel contacts to the carboxy-terminal beta-strand. NOEs between the N-linked glycan and the protein have tentatively been identified.