THE NH2-TERMINAL DOMAIN OF RAT CD2 BINDS RAT CD48 WITH A LOW-AFFINITYAND BINDING DOES NOT REQUIRE GLYCOSYLATION OF CD2

CD2, CD48 and CD58 are structurally similar cell adhesion-molecules fo rming a subset of the immunoglobulin superfamily (IgSF). In humans CD5 8 is a ligand for CD2 while in mice CD2 binds CD48.We constructed a so luble chimeric molecule comprising the extracellular portion of rat CD 48 and domains 3 and 4 of rat CD4 (sCD48-CD4) and used it to examine w hether CD2 is a ligand for CD48 in rats. sCD48-CD4-coated polystyrene Dynabeads(TM) formed rosettes on rat CD2-transfected COS-7 cells, and this rosetting was blocked by anti-CD2 (OX34) and anti-CD48 (OX45) mon oclonal antibodies.We used sucrose-gradient ultracentrifugation to sho w that sCD48-CD4 binds, in solution, to soluble forms of rat CD2 inclu ding the single NH2-terminal IgSF domain of rat CD2 expressed in bacte ria. The upper limit of the affinity of the rat CD48-CD2 interaction i s 4 x 10(5) M-1, lower than the published affinity of human CD2 for CD 58. These results show that rat CD48 binds CD2 on its NH2-terminal IgS F domain with a low affinity and that binding is independent of glycos ylation.