MUTATIONAL ANALYSIS OF THE CD6 BINDING-SITE IN ACTIVATED LEUKOCYTE CELL-ADHESION MOLECULE

The interaction between CD6 and its ligand activated leukocyte cell ad hesion molecule (ALCAM) mediates adhesion of thymocytes to thymic epit helial cells. The extracellular region of ALCAM includes five Ig-like domains, and its N-terminal V-like domain specifically binds to the me mbrane-proximal scavenger receptor cysteine-rich domain of CD6. Previo usly, six ALCAM residues were identified by alanine scanning mutagenes is to contribute to the interaction with CD6. All of these residues ma pped to the predicted A'GFCC'C'' face of ALCAM's N-terminal domain. He re we describe the results of experiments designed to further study th e CD6 binding site. Other mutagenesis experiments at four previously s tudied sites were carried out to better understand their importance fo r the interaction with CD6, and different receptor binding assays were employed to compare the contribution of these and other ALCAM residue s to the CD6-ligand interaction. A total of ten new ALCAM mutants were prepared, and three additional residues were identified as critical f or CD6 binding. These studies have enabled us to classify ALCAM residu es according to their importance for binding and to describe the CD6 b inding site in some detail.