| Peroxisomes are small single-membrane-bounded organelles that play key roles in development and metabolism in most eukaryotic organisms. Peroxisome functions encompass beta-oxidation of fatty acids, detoxification of hydrogen peroxide and the metabolism of a range of biochemical compounds including glyoxylate, glycolate, urate, polyamines, benzoate, phylloquinone, bile acids and plasmalogen to name a few, as well as synthesis of plant hormones such as jasmonic acid (JA) and indole-3-butyric acid (IBA). Peroxisomes exhibit great functional diversity largely because of a plastic proteome that varies greatly depending on the environmental condition, tissue type or developmental stage of the specific organism. Proteins in peroxisomes are imported by the actions of conserved machinery that includes several proteins known as peroxins that are important for peroxisome biogenesis. Proteins destined for the peroxisome matrix contain peroxisome targeting signal (PTS) enabling their recognition by cytosolic receptor proteins that transport them to the peroxisome. Although we have an increased understanding of how peroxisomal protein import is accomplished, we know little about how proteins in peroxisomes are degraded. In this research, I provide evidence that RING domains of three peroxisomal membrane proteins AtPEX2, AtPEX10 and AtPEX12 have E3 ligase activity. I further show that AtPEX2 specifically interacts with two homologous ubiquitin receptor proteins, DSK2a and DS2Kb that have been implicated as adapters linking ubiquitination and 26S proteasome-based degradation events. DSK2 amiRNA lines lacked obvious plant growth phenotypes and were not compromised in peroxisome functions, suggesting that functional redundancies exist among ubiquitin receptor proteins. My results indicate that Arabidopsis RING peroxins and DSK2s can together form a peroxisome membrane associated degradation system.;I also explored the role of a predicted ovarian tumor-like cysteine protease (OCP1) in Arabidopsis. OCP1 was found to be a novel plant specific peroxisomal protein with a canonical C-terminal PTS1 and a novel N-terminal PTS2. Analysis of mutant lines revealed that OCP1 influences IBA metabolism in the peroxisome. Further, ocp1 mutants show retarded degradation of two transiently expressed seedling peroxisome enzymes, isocitrate lyase (ICL) and malate synthase (MS) suggesting that OCP1 has a role in the timely removal of ICL from seedling peroxisomes. In summary, these studies add significantly to our knowledge of proteolysis in plant peroxisomes and open up several avenues for future investigations that may have ramifications in agriculture and biomedical applications. |
