In addition, fibroblasts from patients with defects in peroxisomal boxidation contain enlarged peroxisomes; addition of DHA, an essential PUFA and a major product of peroxisomal boxidation, however, was shown to restore peroxisome morphogenesis. Here we demonstrate that cultivation of COS-7 cells in lipid-free medium promotes an enlargement of peroxisomes giving rise to large spherical organelles reminiscent of peroxisomes observed in fibroblasts from patients with AOX deficiency. Expression of Pex11pb under these conditions resulted in the formation of long membrane protrusions but these asymmetric structures were maintained and proper constriction and division was impaired. We conclude that lipids are required for proper peroxisome morphogenesis and division, and suggest that these processes require a subtle interplay between Pex11pb and membrane lipids. It was proposed that phospholipids directly modulate Pex11pb oligomerization. In this respect, it is possible that the concentration and type of phospholipids within the peroxisomal membrane determines and modulates Pex11pb interaction and thus, the nature of the complexes formed. If we suggest that Pex11pb acts like a scaffold protein, larger Pex11pb complexes might more strongly promote peroxisome elongation than smaller ones. Certain phospholipids might even directly influence Pex11pb structure and positively or negatively regulate self-interaction. Furthermore, Pex11pb has been reported to interact with the membrane adaptors Fis1 and Mff, which are supposed to recruit the fission GTPase DLP1 to the peroxisomal membrane. Fis1 and Mff are both suggested to form homo-dimers,, indicating that the formation of even larger complexes may modulate peroxisome fission. However, their preservation and detection will likely vary depending on the solubilization conditions applied. We would like to note that neither Fis1 nor Mff were found to co-migrate with the 56 kDa band of Pex11pb-Myc in immunoblots, further supporting Pex11pb-Myc dimer formation. We as well demonstrated that the N-terminal cysteines C18, C25, and C85 are not essential for peroxisomal membrane elongation. It is unlikely that these cysteines contribute to dimer formation by covalent bonds, as Pex11pb self-interactions in coimmunoprecipitation studies are lost in the Tulathromycin B presence of Triton X100. In line with this, no effect on membrane elongation properties of Pex11pb was observed in the absence of all three cysteines. It is possible that transient, intramolecular disulfide bridges exist which may stabilize Pex11pb structure or protein interactions later on during the division process. In addition, no alterations in peroxisome elongation or division were detected when the putative N-terminal phosphorylation sites S11 and S38 within the first 40aa of Pex11pb were altered generating putative phospho-mimicking “on” and “off” mutants. Although phosphorylation at other putative sites within Pex11pb is possible, we currently do not have experimental evidence that Pex11pb is Folinic acid calcium salt pentahydrate phosphorylated under our experimental conditions. The regulation of Pex11p activity by phosphorylation has recently been demonstrated for ScPex11p and PpPex11p,, however, the phosphorylation sites are not conserved among organisms. It is possible that one of the other mammalian Pex11 proteins is phosphorylated, or that other diverse regulatory mechanisms have evolved. Thus, functional and regulatory differences as well as distinct biochemical properties should be considered when investigating Pex11 isoforms or proteins from different species.