Equilibrium and kinetic studies of the association of catalytic and regulatory subunits of aspartate transcarbamoylase.

I'*'I-Labeled catalytic and regulatory subunits were used to study the assembly of aspartate transcarbamoylase (EC 2.1.3.2;carbamoylphosphate: L-aspartate carbamoyltransferase) of Escherichia coli.The association of the two catalytic and three regulatory subunits in the oligomeric enzyme is virtually irreversible as shown by the small rate constant (less than 2 X lo-' s-') for exchange of a subunit.With labeled subunits of high specific radioactivity the pathways of assembly could be studied at protein concentrations in the nanomolar range, and the overall process which required about 20 min could be "stopped" at various times by a "chase" with unlabeled subunits.Various intermediates were "trapped" and their amounts measured after electrophoretic separation.