beta-Galactosidase alpha-complementation. Overlapping sequences.

Enzyme activity is restored to two defective beta-galactosidase molecules (M15 protein lacking amino acid residues 11-41 and M112 protein lacking residues 23-31) by incubation with CNBr2 (residues 3-92 of beta-galactosidase). M15 and M112 proteins (alpha-acceptors) are dimers. Complemented enzyme, like wild type, has a tetrameric structure. Cleavage of CNBr2 with glutamic acid-specific protease yielded a much smaller alp ha-donor (3-41 peptide) which was also effective in complementation, indicating that the M15 protein can supply all of the residues from 42-92 for the structure of complemented enzyme. Treatment of M112 protein/3-41 peptide complemented enzyme with trypsin under very mild conditions followed by examination of the products demonstrated that the alpha-donor pep]tide supplies the NH2-terminal segment of complemented enzyme. Similar trypsin treatment of M15 protein/CNBr2 indicated that in this complemented enzyme the polypeptide region beyond those residues missing in the alpha-acceptor can be provided by either the alpha-donor or the alpha-acceptor. Both M15 protein and M112 protein are more susceptible to mild tryptic proteolysis than complemented enzyme, indicating a more open structure. Several antipeptide antibodies that react with these two proteins do not react with beta-galactosidase. M112 protein, like M15 protein, can be activated by anti-beta-galactosidase but to a much higher level.