Fig. 8. Schematic model for the morphogenesis of mammalian peroxisomes, based on observations reported here and previously (Schrader et al., 1996; Schrader et al., 1998b; Schrader et al., 2000). Recent evidence suggesting the involvement of the ER in the de novo formation of peroxisomes (Titorenko and Rachubinski, 2001; Eckert and Erdmann, 2003; Lazarow, 2003) is included. The majority of matrix and membrane proteins are synthesized on free ribosomes in the cytosol and imported post-translationally into pre-existing peroxisomes. Peroxisomes multiply by budding and segmentation from pre-existing ones. Under induced conditions (e.g. cultures at low density, growth factors, fatty acids, free radicals), highly elongated peroxisomes are formed that undergo segmentation and fission, forming spherical peroxisomes. Pex11pß is involved in the elongation/tubulation of peroxisomes (Schrader et al., 1998b), whereas DLP1 mediates peroxisomal fission (Koch et al., 2003; Li and Gould, 2003). Proteins mediating the constriction of peroxisomes are presently unknown. Proper intracellular distribution of the formed peroxisomes requires microtubules and a functional dynein/dynactin motor (Schrader et al., 2003). In yeast and plants peroxisomes are distributed via the actin cytoskeleton (Hoepfner et al., 2001; Mathur et al., 2002).