Structural and functional characterization of the CD2 immunoadhesion domain. Evidence for inclusion of CD2 in an alpha-beta protein folding class.

The T-lymphocyte transmembrane glycoprotein CD2 plays an important physiological role in facilitating adhesion between T-lymphocytes and their cognate cellular partners. This interaction is mediated by binding of CD2 to the broadly distributed surface polypeptide LFA-3 and augments the recognition function of the CD3-Ti antigen-major histocompatibility complex receptor via stabilization of conjugate formation between cells. To define better the structural components of the CD2 extracellular region which are important in contact-mediated cellular adhesion, a single-domain CD2 immunoadhesion protein has been prepared from papain digestion of a soluble two-domain CD2 molecule. This amino-terminal domain fragment binds to LFA-3 on human B-cells with a dissociation constant of 0.4 microM, possesses functional immunoadhesion epitopes as defined by the binding of monoclonal antibodies raised to native CD2, and retains the ability to inhibit sheep erythrocyte rosette formation with human T-cells. Thus, all of the immunoadhesion functions ascribed to CD2 reside within the amino-terminal domain. Circular dichroism analysis of the isolated CD2 adhesion domain suggests the presence of substantial alpha-helical character (22%), consistent with earlier computer modeling analyses that predicted a pattern of alternating alpha-helices and beta-sheets within the extracellular region of CD2. Despite the existence of short stretches of sequence homology between CD2 and immunoglobulin superfamily members, the circular dichroism data provide supporting biophysical evidence for classification of CD2 in an alpha-beta (either alpha/beta or alpha + beta) protein folding class.