Insights into peroxisome matrix protein import and IBA metabolism through analysis of IBA-response mutants
Braam, Janet; Bartel, Bonnie
Doctor of Philosophy
Peroxisomes are single membrane-bound organelles that function to compartmentalize certain metabolic reactions critical to plant and animal development. I have studied peroxisomal processes in the model plant Arabidopsis thaliana , with a focus on the import of matrix proteins from the cytoplasm into the organelle matrix and the metabolism of the plant hormone indole-3-butryic acid (IBA). In this thesis, I describe my characterization of Arabidopsis thaliana peroxisome defective mutants isolated through forward and reverse genetic screens in physiological and biochemical assays. Peroxisome import depends on more than a dozen pero xin (PEX) proteins, with PEX5 and PEX7 serving as receptors that shuttle proteins bearing a peroxisome t argeting sequence (PTS) into the organelle. PEX5 is the PTS1 receptor, PEX7 is the PTS2 receptor, and in both plants and mammals, PEX7 depends upon PEX5 binding to deliver PTS2 cargo into the peroxisome. I found a pex7 missense mutation, pex7-2, that disrupts PEX7-cargo binding and PEX7-PEX5 interactions in yeast, as well as PEX7 accumulation in plants. I examined localization of peroxisomally-targeted GFP derivatives in light-grown pex7 mutants and, surprisingly, observed defects not only in PTS2 import, but also in PTS1 protein import. These PTS1 import defects were accompanied by a decrease in PEX5 accumulation in light-grown pex7 mutants. Together, these data suggest that PEX5 and PTS1 import depend on the PEX7 PTS2 receptor in Arabidopsis and reveal a role for the environment in modulating peroxin function. Genetic evidence suggests that indole-3-butyric acid (IBA) is converted to the auxin indole-3-acetic acid (IAA) in Arabidopsis peroxisomes. The IBR1, IBR3, and IBR10 proteins contain peroxisomal targeting signals and are candidates for catalyzing the various steps of IBA beta-oxidation. My analysis of the ibr mutants has provided evidence for the importance of the IBR enzymes and insight into the roles of IAA that derives from IBA beta-oxidation. In humans, deficiencies in peroxins underlie the peroxisomal biogenesis disorders, which are frequently lethal in early infancy. Advancing our understanding of peroxisome biogenesis and metabolism in a genetically distinct model system will allow the continued refinement of our understanding of these essential organelles.