Biotin induces tetramerization of a recombinant monomeric avidin - A modelfor protein-protein interactions

Chicken avidin, a homotetramer that binds four molecules of biotin was conv erted to a monomeric form by successive mutations of interface residues to alanine, The major contribution to monomer formation was the mutation of tw o aspartic acid residues, which together account for ten hydrogen bonding i nteractions at the 1-4 interface, Mutation of these residues, together with the three hydrophobic residues at the 1-3 interface, led to stable monomer formation in the absence of biotin. Upon addition of biotin, the monomeric avidin reassociated to the tetramer, which exhibited properties similar to those of native avidin, with respect to biotin binding, thermostability, a nd protease resistance. To our knowledge, these unexpected results represen t the first example of a small monovalent ligand that induces oligomerizati on of a monomeric protein. This study may suggest a biological role for low molecular weight ligands in inducing oligomerization and in maintaining th e stability of multimeric protein assemblies.