High-Level Expression of a Mitochondrial Enzyme, Ornithine Transcarbamylase from Rat Liver, in a Baculovirus Expression System

The mitochondrial enzyme, ornithine transcarbamylase (OTC) from rat liver was expressed in Spodoptera frugiperda (Sf) insect cells using a baculovirus vector. When insect cells were infected with recombinant Autographica californica nuclear polyhedrosis virus (AcNPV) containing a cDNA encoding the precursor form of OTC (pOTC) inserted into the polyhedrin gene, they expressed catalytically active enzyme at levels of approximately 2.5 μg/10⁶ cells. About 25% of the active enzyme was a novel, partially processed product of pOTC containing four extra amino acids at the amino terminus of OTC. The most abundant protein found in mitochondria from infected insect cells was the normal processing intermediate iOTC, which contains 8 extra amino acids at the amino terminus of OTC. Whereas this species, present at 20 μg/10⁶ cells, was not active and did not bind the transition-state analog inhibitor of OTC, δ-PALO, the novel processing product did bind and was affinity-purified, along with mature OTC, on a PALO-affinity column. The OTC expressed in insect cells was located in the same compartment of the mitochondrion as in rat liver. The incomplete processing occurred in vitro in both noninfected and infected insect cells. The high level of expression of iOTC using the baculoviral expression system provides a means of overproducing an obligatory intermediate in the mitochondrial import process.