In eukaryotes, pyruvate, a key metabolite produced by glycolysis, is converted by a mitochondrial pyruvate dehydrogenase complex (PDH) to acetyl-coenzyme A, which is fed into the tricarboxylic acid cycle. Two additional enzyme complexes catalyze similar oxidative decarboxylation reactions albeit using different substrates, the branched-chain ketoacid dehydrogenase (BCKDH) and the 2-oxoglutarate dehydrogenase (OGDH). In diplonemids, one of the most abundant and diverse groups of oceanic protists, comparative transcriptome analyses indicated that their PDH complex is compositionally unique, with the conventional E1 subunit replaced by an aceE protein of prokaryotic origin. Here we endogenously tagged the common E3 subunit with a protein A tag and performed immunoprecipitation experiments.