Yarrowia lipolytica, a yeast genetic system to study mitochondrial complex I

The obligate aerobic yeast Yarrowia lipolytica is introduced as a powerful new model for the structural and functional analysis of mitochondrial complex I. A brief introduction into the biology and the genetics of this nonconventional yeast is given and the relevant genetic tools that have been developed in recent years are summarized. The respiratory chain of Y. lipolytica contains complexes I-IV, one "alternative" NADH-dehydrogenase (NDH2) and a non-heme alternative oxidase (AOX). Because the NADH binding site of NDH2 faces the mitochondrial intermembrane space rather than the matrix, complex I is an essential enzyme in Y. lipolytica. Nevertheless, complex I deletion strains could be generated by attaching the targeting sequence of a matrix protein, thereby redirecting NDH2 to the matrix side. Deletion strains for several complex I subunits have been constructed that can be complemented by shuttle plasmids carrying the deleted gene. Attachment of a hexa-histidine tag to the NUGM (30 kDa) subunit allows fast and efficient purification of complex I from Y. lipolytica by affinity-chromatography. The purified complex has lost most of its NADH:ubiquinone oxidoreductase activity, but is almost fully reactivated by adding 400-500 molecules of phosphatidylcholine per complex I. The established set of genetic tools has proven useful for the site-directed mutagenesis of individual subunits of Y. lipolytica complex I. Characterization of a number of mutations already allowed for the identification of several functionally important amino acids, demonstrating the usefulness of this approach.