The catalytic activity and the state of aggregation of the dihydrolipoyl transacetylase-lipoamide dehydrogenase binding protein (E2-E3BP) subcomplex of the bovine heart pyruvate dehydrogenase multienzyme complex were investigated. Treatment of E2-E3BP with the chaotropic salts GndnCl or KSCN led to a rapid decrease in transacetylase activity which was accompanied by a loss of the native quaternary structure, as indicated by changes in the sedimentation properties of the E2-E3BP subcomplex. Reassembly or refolding of dissociated E2-E3BP was achieved for the GndnCl-treated subcomplex using a defined protocol. This reassembly procedure effectively excluded all E3BP from the reassembled oligomeric transacetylase. The reassembled oligomeric E2, free of E3BP, was unable to reconstitute the overall activity of the complex following incubation with pyruvate dehydrogenase (E1) and lipoamide dehydrogenase (E3). In binding studies using radiolabeled components it was demonstrated that the reassembled transacetylase, while retaining its capacity for reductive acetylation and its ability to bind E1, lost its ability to bind E3. The evidence presented in this study indicates that the strong association of E3BP with E2 facilitates the binding of E3, the lipoamide dehydrogenase component, and therefore may have an important role in the assembly and ultimately the catalytic activity of the pyruvate dehydrogenase multienzyme complex.