Chen, Ho Ann; (2000) Biophysical characterisation of domain 1 of rat CD2. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

CD2, a glycoprotein found on the T-lymphocyte, plays an important role in mediating the adhesion of T-lymphocytes to its accessory and target cells. The ligand-binding surface of CD2, which is located on the N-terminal domain of CD2 (CD2d1), has an unusually high proportion of charged residues. The ionic interactions of these charged residues are though to play a significant role in defining the ligand-specificity and binding affinity of CD2 with its ligands CD48 and CD58. The determination of the electrostatic properties of these proteins can therefore contribute to our understanding of the structure-activity relationships for these adhesion complexes. In this thesis, the biophysical characterisation of the electrostatic properties of CD2d1 is described. The principal method used for the investigation is nuclear magnetic resonance (NMR) which permit the accurate determination of the ionisation constants of all the individual acidic residues. The characterisation of the binding interaction involved site-directed mutagenesis of these residues on the binding surface. In addition, the dynamic properties of CD2d1 are also investigated by NMR relaxation experiments. The significance of the finding are discussed The pH titration of CD2d1 revealed a glutamate (Glu41) on the binding surface that has an anomalously high pka. This anomalous pka has an extensive effect on the chemical shift that suggests protein self-association mediated by this residue. This self-association was confirmed by relaxation analysis, and the CD2 was shown to dimerise with a very low affinity, but this dimerisation is nevertheless highly specific and has a pronounced effect on the relaxation parameters. The results indicated that CD2 dimerisation is maximal when Glu41 is protonated and Glu29 is deprotonated. The implication of the findings to the analysis of dynamics by NMR in discussed, and the significance of the observations to recent structural and functional analyses of rat CD2 interaction with CD48 is also examined.

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