Determining the Physiological Significance of a Rab GAP (GTPase Accelerating Protein) Found at Peroxisomes in Yeast

Rab GTPases (Rabs) are lipid-anchored proteins that regulate membrane docking and fusion of vesicles and membrane-bound organelles. Rabs change activity based on their association with GTP (active) or GDP (inactive). Rabs require accessory proteins called Rab GAPs (GTPase accelerating proteins) to accelerate GTP hydrolysis and inactivate Rab signal. We have identified a Rab GAP, Gyp8, which localizes to peroxisome membranes in the yeast S. cerevisiae. This is the first report in any experimental system of a Rab GAP localized to peroxisomes, demanding an investigation of possible physiological roles of Gyp8 in regulating peroxisome dynamics. Cells lacking Gyp8 suffer no overt defects in peroxisome function or cargo transport, as fluorescence microscopy indicates that gyp8 mutants possess peroxisomes that correctly localize peroxisome cargo proteins. Studies of peroxisome cargo import using sensitive, quantitative colorimetric assays are ongoing. To examine possible roles of Gyp8 in peroxisome creation or destruction, cells were monitored when fed oleic acid media requiring peroxisome function for cell viability and growth. Cells lacking Gyp8 saturated their growth at a lower density than wild type cells, producing microbial growth curves that suggested a lack of diauxic shift or other functional adaptation when all oleic acid had been consumed. When oleic acid is depleted, yeast are known to rapidly destroy their surplus peroxisomes. We hypothesize that cells lacking Gyp8 may suffer slower kinetics of peroxisome destruction via pexophagy. Ongoing studies address the kinetics of pexophagy using luciferase reporter assays in the presence, absence and overabundance of Gyp8