title: beta-Galactosidase alpha-complementation. A model of protein-protein interaction.
abstract: Studies on beta-galactosidase alpha-complementation are reviewed. The isolation and structure of two beta-galactosidase fragments that form an enzymically active complex are described. One of these is a cyanogen bromide peptide from whole beta-galactosidase; the other is a dimeric protein from a lacZ deletion mutant of Escherichia coli. The mechanism most likely involves an initial binding of two cyanogen bromide peptides to the dimer, followed by formation of a tetramer, and finally a slow conformational change of the complex to a native-like enzyme. The overall reaction is essentially irreversible. A region of the polypeptide chain involved in dimer-dimer contact must be supplied by the cyanogen bromide peptide. alpha-Complemented enzyme contains overlapping sequences. Proteolytic experiments were carried out to determine the origin of the functionally important segment. The effect on alpha-complementation of amino acid substitutions at four positions in the polypeptide chain was investigated. The implications of these results for beta-galactosidase structure and for proteins in general are discussed.
mesh_terms: D000595:Amino Acid Sequence; D004926:Escherichia coli / Q000201:enzymology; D005696:Galactosidases / Q000235:genetics*; D005816:Genetic Complementation Test; D046911:Macromolecular Substances; D011487:Protein Conformation; D013329:Structure-Activity Relationship; D001616:beta-Galactosidase / Q000235:genetics*
chemical_list: D046911:Macromolecular Substances; D005696:Galactosidases; D001616:beta-Galactosidase