PROTEIN-PROTEIN INTERACTIONS IN THE PYRUVATE-DEHYDROGENASE MULTIENZYME COMPLEX - DIHYDROLIPOAMIDE DEHYDROGENASE COMPLEXED WITH THE BINDING DOMAIN OF DIHYDROLIPOAMIDE ACETYLTRANSFERASE

Background: The ubiquitous pyruvate dehydrogenase multienzyme complex is built around an octahedral or icosahedral core of dihydrolipoamide acetyltransferase (E2) chains, to which multiple copies of pyruvate de carboxylase (E1) and dihydrolipoamide dehydrogenase (E3) bind tightly but non-covalently. E2 is a flexible multidomain protein that mediates interactions with E1 and E3 through a remarkably small binding domain (E2BD). Results: In the Bacillus stearothermophilus complex, the E2 G ore is an icosahedral assembly of 60 E2 chains. The crystal structure of the E3 dimer (101 kDa) complexed with E2BD (4 kDa) has been solved to 2.6 Angstrom resolution. Interactions between E3 and E2BD are domin ated by an electrostatic zipper formed by Arg135 and Arg139 in the N-t erminal helix of E2BD and Asp344 and Glu431 of one of the monomers of E3, E2BD interacts with both E3 monomers, but the binding site is loca ted close to the twofold axis. Thus, in agreement with earlier biochem ical results, it is impossible for two molecules of E2BD to bind simul taneously to one E3 dimer, Conclusions: Combining this new structure f or the E3-E2BD complex with previously determined structures of the E2 catalytic domain and the E2 lipoyl domain creates a model of the E2 c ore showing how the lipoyl domain can move between the active sites of E2 and E3 in the multienzyme complex.