ASSEMBLY OF MITOCHONDRIAL SUPERCOMPLEXES IN THE YEAST SACCHAROMYCES CEREVISIAE
Zoe Cosner, Flavia Fontanesi, Antoni Barrientos.
University of Miami, Miami, FL.
The mitochondrial oxidative phosphorylation system, composed of mitochondrial respiratory chain (MRC) complexes I-IV, ATP synthase, and two electron carriers, provides most cellular energy in the form of ATP. MRC complexes form ordered structures called supercomplexes, which have been proposed to enhance individual complex stability and increase respiratory efficiency. Supercomplex organization is well characterized in lower to higher eukaryotes. However, despite their importance, supercomplex assembly remains poorly understood. Several observations in yeast and mammalian cells have suggested that structural subunits of different complexes may interact in subassembly intermediates. We hypothesize that supercomplexes do not originate from the association of preassembled individual complexes. Rather, their biogenesis involves the incorporation of partially assembled complexes and free subunits into subassembly intermediates of increasing complexity. To investigate the molecular mechanisms involved in supercomplex assembly, we use the yeast Saccharomyces cerevisiae as a model system. Yeast supercomplexes are composed of a complex III dimer and 1 or 2 complex IV monomers. We have engineered a system that allows for regulatable expression of CIII and CIV subunits. Our novel yeast models allow us to halt a specific MRC subunit's expression and restore it at any moment after the MRC enzymes have been turned over, thus facilitating the study of MRC supercomplex assembly pathways de novo. Our initial data show that some CIII and CIV subunits are incorporated directly into supercomplexes in late assembly stages, while others accumulate in earlier subassemblies, which could represent supercomplex assembly intermediates. We are now in the process of further characterizing the step-by-step supercomplex assembly pathway.