A hybrid proteolytic fragment of Escherichia coli aspartokinase I-homoserine dehydrogenase I. Structure, inhibition pattern, dissociation properties, and generation of two homodimers.

A hybrid dimeric fragment of Escherichia coli aspartokinase I-homoserine dehydrogenase I (Fazel, A., Müller, K., Le Bras, G., Garel, J.-R., Véron, M., and Cohen, G. N. (1983) Biochemistry 22, 158-165) has been purified and shown to possess both aspartokinase and homoserine dehydrogenase activities and is rather stable in the presence of L-threonine. Its two activities are still inhibited by threonine, but noncooperatively in contrast to the native protein. The aspartokinase activity is found to be more sensitive to threonine than the dehydrogenase activity. In the absence of threonine, the different chains of the hybrid (Mr = 89,000 + 59,000) dissociate first into monomers, this being followed by the pairing of two homologous chains to form two homodimers. In the presence of L-threonine, the two homodimers do not dissociate to re-form the hybrid fragment. The NH2-terminal analysis of different chains of the hybrid shows that the homodimers correspond, respectively, to the dimer of the native protein (Mr = 2 X 89,000) and to a dimer already described (Véron, M., Falcoz-Kelly, F., and Cohen, G. N. (1972) Eur. J. Biochem. 28, 520-527).