Ubiquitination on the Peroxisomal Membrane for Protein Transport in Plants

Abstract

Peroxisomes are ubiquitous organelles present in most eukaryotic cells that have important biological functions related to fatty acid metabolism and detoxification of reactive oxygen species. Disruption of peroxisomal function affects the survival of cells and organisms. Peroxisomes do not have their own genome, and peroxisomal proteins are encoded in the nuclear genome. Therefore, efficient and accurate posttranslational transport of peroxisomal proteins is necessary to maintain peroxisomal function. In mammals, yeast, and plants, many factors involved in protein transport to peroxisomes have been identified and their molecular mechanisms elucidated. In plants, analysis of Arabidopsis peroxisome mutants, such as apem (aberrant peroxisome morphology) and ibr (indole-3-butyric acid-response), enabled the identification of the factors mediating protein transport. Of these, several proteins, such as PEX1 (Peroxin 1), PEX2, PEX4, PEX6, PEX10, PEX12, PEX22, and APEM9, constitute the ubiquitin system on the peroxisomal membrane, and loss of function of each protein reduces the efficiency of protein transport to peroxisomes. This ubiquitin-dependent peroxisomal protein transport system is also present in yeast and mammalian cells and is an example of a type of ubiquitin modification that serves as a signaling tag rather than as a tag for protein degradation. This chapter introduces the factors involved in protein transport to the peroxisome via the ubiquitin system in plants and outlines their functions.