Structural biology of the cell adhesion protein CD2: from molecular recognition to protein folding and design.

Abstract

CD2 (cluster of differentiation 2) is a cell adhesion molecule expressed on T cells and is recognized as a target for CD48 (rats) and CD58 (humans). Tremendous progress has been achieved in understanding the function of CD2, the mechanism of molecular recognition and protein folding, thus, leading towards the use of this protein as a scaffold for protein design. CD2 has been shown to set quantitative thresholds in T cell activation both in vivo and in vitro. Further, intracellular CD2 signaling pathways and networks are being discovered by the identification of several cytosolic tail binding proteins. In addition, a new method for directly measuring heterophilic adhesion has been developed. The functional "hot spot" for the adhesion surface of CD2 and CD58 has been dissected. Detailed NMR studies reveal that rat CD2 weakly self-associates to form a homodimeric structure in solution. Dynamic interaction of CD2 with the GYF and SH3 domains has been investigated. CD2 has been shown to form fibrils in the presence of 2,2,2-trifluoroethanol (TFE) and at low pH. Furthermore, kinetic studies have been completed to monitor the effect of surface hydrophobic residues and intramolecular bridges on the folding pathways of CD2. Our lab has de novo designed single calcium-binding sites into domain 1 of rat CD2 (CD2-D1) with strong metal selectivity. In addition, the EF-hand motifs have been grafted into CD2 to understand the site-specific calcium-binding affinity of calmodulin and calcium-dependent cell adhesion.