Peroxisomes are eukaryotic cellular organelles that are involved in different catabolic and anabolic functions like α- and β-oxidation of very long chained and branched fatty acids, ether lipid synthesis and catabolism of purins and polyamines. The generated hydrogenperoxide is degraded in the peroxisomes by the enzyme catalase. In 1954, microbodies were described for the first time by the Swedish doctoral student Johannes Rhodin and were named peroxisomes in 1966 by de Duve. These highly dynamic organelles are present in all types of eukaryotic cells and their size, form, number per cell and protein contents can be quite variable depending on the organism, cell type and metabolic needs.
In eukaryotic cells peroxisomes multiply by asymmetric fission or can be formed de novo.
Proteins, which are involved in peroxisomal biogenesis, are called peroxins (PEX). The first step of peroxisomal de novo synthesis involves the formation of the peroxisomal membrane followed by the insertion of further peroxisomal membrane proteins and matrix proteins into the organelle. Mutations in the genes coding for the “early” peroxins PEX3, PEX16 and PEX19 lead to loss of any detectable peroxisomal membrane structures. The finding that fully functional peroxisomes are synthesized in PEX3, PEX16 and PEX19 lacking cell lines upon transfection of the corresponding cDNA raises the question about the origin of the peroxisomal membrane.
Several experiments suggest the involvement of vesicle formation at the endoplasmic reticulum (ER). After insertion of some peroxins into the ER, preperoxisomal vesicles are budded from a specialized region of the ER. Peroxisomal membrane and matrix proteins are postranslationally inserted into these peroxisomal vesicles until mature peroxisomes are generated.
The early peroxins PEX3, PEX16 and PEX19 are also essential for posttranslational insertion of peroxisomal membrane proteins (PMPs) into the peroxisomal membrane. Most PMPs have a quite variable membrane targeting sequence (mPTS) and are bound in the cytosol after translation by their receptor PEX19. The cargo-loaded PEX19 binds to the import-receptor PEX3 at the peroxisomal membrane and the PMP is imported. Afterwards PEX19 is recycled into the cytosol for another PMP import cycle.
We were able to crystallize the cytosolic part of PEX3 in combination with N-terminal parts of PEX19 and could therefore improve the understanding of the PEX3-PEX19-PMP binding mechanism. Structural analysis revealed three highly conserved patches within PEX3: The PEX19-binding region on top of a α-helical bundle, a hydrophobic groove potentially involved in PMP binding and an obviously non-functional acidic cluster.
In contrast to other organelles like the endoplasmic reticulum and mitochondria, which import unfolded proteins, peroxisomes are able to import fully folded, co-factor bound proteins and protein complexes. Peroxisomal matrix proteins exhibit the peroxisomal targeting signals PTS1 or PTS2, which are recognized by the soluble receptors PEX5 and PEX7 in mammalian cells. The most common targeting signal PTS1 represents a C-terminal tripeptide with the sequence Ser/Cys/Ala-Lys/Arg/His-Leu (SKL) and is bound by the PTS1 receptor PEX5. After binding the receptor-cargo complex is associated at the peroxisomal membrane via interaction of the PTS-receptors with the docking complex. This complex is composed of the integral membrane proteins PEX13 and PEX14. The docking complex interacts with the RING (really interesting new gene) finger complex consisting of PEX2, PEX10 and PEX12 to build the importomer. A highly dynamic, size-adaptable transient import pore is formed by the receptor PEX5 and the peroxin PEX14. After cargo release the receptors PEX5 and PEX7 are recycled back into the cytosol using ATP. This export step requires monoubiquitination of PEX5 and the export machinery composed of PEX1, PEX6 and its membrane-recruiting factor PEX26. Currently we are working on the mechanisms underlying the export of the cycling PTS1-receptor PEX5.