The dihydrolipoyl acyltransferase (BCE2) subunit of the plant branched-chain alpha-ketoacid dehydrogenase complex forms a 24-mer core with octagonal symmetry

Little is known of the plant branched-chain cr-ketoacid dehydrogenase compl ex. We have undertaken a detailed study of the structure of the dihydrolipo yl acyltransferase (BCE2) subunit that forms the core of the complex, to wh ich two other enzymes attach. Mature Arabidopsis thaliana BCE2 was expresse d in Escherichia coli. The soluble recombinant protein was purified using a Superose 6 size-exclusion column to >90% homogeneity and was catalytically active. The recombinant protein formed a stable complex with a native mole cular mass of 0.95 MDa and an S coefficient of 19.4, consistent with format ion of a 24-mar. Negative-staining transmission electron microscopy of the recombinant protein confirmed that BCE2 forms a core with octagonal symmetr y. Despite divergence of mammalian and plant BCE2s, there is clearly conser vation of structure that is independent of primary sequence.