Ubi complex characterization

Ubiquinone (UQ) is a polyprenylated lipid that is conserved from bacteria to humans and is crucial to cellular respiration. How the cell orchestrates the efficient synthesis of UQ, which involves the modification of extremely hydrophobic substrates by multiple sequential enzymes, remains an unresolved issue. Here, we demonstrate that seven Ubi proteins form the Ubi complex, a stable metabolon that catalyzes the last six reactions of the UQ biosynthetic pathway in Escherichia coli. In addition to five Ubi enzymes, the 1 MDa Ubi complex contains the newly identified UbiJ and UbiK proteins that we show to be involved in the binding of UQ intermediates and the structuration of the multiprotein complex. We also develop an in vitro assay and show that UQ biosynthesis occurs in the cytoplasmic fraction of E. coli, from which we purify the Ubi complex. Collectively, our work documents a rare case of stable metabolon and revisits the current thinking that the hydrophobic intermediates of the UQ pathway are metabolized in association with the membrane.