The tryptophan synthase α2β2 complex: A comparison of the reactivity of amino groups in the α and β2 subunits and in the complex by differential labeling studies

Abstract

The interaction of the α and β 2 subunits of tryptophan synthase of Escherichia coli to form an α 2 β 2 complex has been probed by differential labeling studies. In the first step the separate α or β 2 subunit or the α 2 β 2 complex was labeled by reductive methylation with trace amounts of [ 3H]HCHO in the presence of NaCNBH 3. In the second step the 3H-labeled preparation was fully labeled under denaturing conditions with [ 14C]HCHO and NaCNBH 3. Peptides containing labeled monomethyl or dimethyl amino groups were isolated after thermolytic digestion or after cyanogen bromide treatment. The 3 H 14 C ratio of each peptide is a measure of the relative reactivity of the amino group or groups in each peptide. The most reactive amino group in the α subunit, lysine-109, is strongly shielded from modification in the α 2 β 2 complex. The most reactive amino group in the β 2 subunit, the amino-terminal threonine, is not shielded from modification in the α 2 β 2 complex.