Required for ubiquinone (coenzyme Q) biosynthesis. Binds hydrophobic ubiquinone biosynthetic intermediates via its SCP2 domain and is essential for the stability of the Ubi complex. May constitute a docking platform where Ubi enzymes assemble and access their SCP2-bound polyprenyl substrates. Cofactor biosynthesis; ubiquinone biosynthesis. Belongs to the UbiJ family.