Molecular model of the N-terminal receptor-binding domain of the human CD6 ligand ALCAM.

CD6-ligand interactions have been implicated in the regulation of T-cell adhesion and activation. CD6 is a member of the scavenger receptor family, whereas its human ligand (ALCAM) belongs to the immunoglobulin superfamily. The extracellular region of ALCAM includes five immunoglobulin-like domains. As a fusion protein, the N-terminal extracellular domain of ALCAM (ALCAMD1) binds specifically to CD6. We report the construction, assessment, and analysis of a molecular model of ALCAMD1. The model defines the CDR-analogous loops, the location of N-linked glycosylation sites, and residues that form the beta-sheet faces of the immunoglobulin-like domain. Predicted structural characteristics of the A'GFCC'C" face of the model are consistent with the presence of monomeric and dimeric forms of ALCAMD1, which has implications for the receptor-ligand interactions.