Abstract
The mitochondrial citric acid cycle is a central hub of cellular metabolism, providing intermediates for biosynthetic pathways and channeling electrons to the respiratory chain complexes. In this study, we elucidated the composition and organization of the multienzyme complex α-ketoglutarate dehydrogenase (α-KGDH). In addition to the three classical E1-E3 subunits, we identified a novel component, Kgd4 (Ymr31/MRPS36), which was previously assigned to be a subunit of the mitochondrial ribosome. Biochemical analyses demonstrate that this protein plays an evolutionarily conserved role in the organization of mitochondrial α-KGDH complexes of fungi and animals. By binding to both the E1-E2 core and the E3 subunit, Kgd4 acts as a molecular adaptor that is necessary to a form a stable α-KGDH enzyme complex. Our work thus reveals a novel subunit of a key citric acid-cycle enzyme and shows how this large complex is organized.