Peroxisystem – using systematic tools to identify new peroxisomal proteins

Peroxisomes are hubs of cellular metabolism, performing essential cellular functions such as α- and β-oxidation of fatty acids, amino acid synthesis and metabolism of Reactive Oxygen Species. Impairment of the metabolic functions of peroxisomes, not to mention their complete biogenesis, leads to severe diseases that often cause premature death. Although peroxisomes were identified more than 60 years ago, their complete repertoire of proteins is still a mystery. We developed systematic tools based on high content microscopy to uncover the peroxisome proteome. To fully characterize the proteome we created a collection of Saccharomyces cerevisiae strains in which each protein is N’ tagged with a GFP fluorophore. We integrated a peroxisomal marker into this entire collection of ~6000 strains covering the entire yeast proteome and identified more than 40 new peroxisomal proteins, many of which are conserved to humans. Studying these proteins enabled us to unravel novel peroxisomal functions, targeting signals and targeting machineries to peroxisomes. We identified a new peroxisome targeting receptor, that we named Pex9, that is functional under specific metabolic conditions and is targeting a unique set of matrix proteins. Additionally, we identified proteins that are targeted to peroxisomes by non-canonical targeting signals and targeting machineries. One of these proteins is Malate Dehydrogenase 2 (Mdh2) that is targeted to peroxisomes via association with Mdh3 and a novel Pex5-dependent piggyback mechanism. The systematic tools that we have developed and are continuously developing in the lab are constantly shedding new light on the fascinating nature of peroxisomes and peroxisomal disorders.