The peroxisomal exportomer

Abstract

Peroxisomes are ubiquitous cell organelles of eukaryotic cells. Depending on environmental changes and cellular demands, peroxisomes display a high plasticity in metabolic functions. A prerequisite to carry out their physiological tasks is compartmentalization of peroxisomal enzymes in the lumen of this organelle, the peroxisomal matrix. The matrix proteins are synthesized on free polyribosomes in the cytosol and harbor a peroxisomal targeting sequence (PTS). They are targeted to the peroxisomal membrane by soluble PTS-receptors. Following the release of the cargo enzyme into the peroxisomal matrix, the PTS-receptor is ubiquitinated and exported back to the cytosol to facilitate further rounds of matrix protein import. The retrotranslocation of the receptor is facilitated by a molecular machinery that comprises enzymes required for the ubiquitination as well as for the ATP-dependent extraction of the receptor from the membrane. Furthermore, recent evidence indicates that the export machinery of the receptors might function as molecular motor not only for the retrotranslocation of the receptors themselves but also for the import of peroxisomal matrix proteins. This is thought to be achieved by coupling the ATP-dependent removal of the PTS-receptor with the cargo protein translocation into the organelle. In this review, we will discuss the combined data on the architecture and molecular function of the peroxisomal receptor export machinery, the peroxisomal exportomer.