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The cover shows an electronmicrograph of a mouse dendritic cell with a cluster of peroxisomes (P) in close association with lamellar structures (L) that contain a characteristic central stripe. (CP, clathrin-coated pit; ER, endoplasmic reticulum; M, mitochondrion; PM, plasma membrane). By means of the cryo-immunogold technique, these lamellae have recently been demonstrated to have membrane continuities with the ER and to harbor the peroxisomal membrane proteins Pex13p and PMP70 (Geuze, H.J., Murk, J.L., Stroobants, A.K., Griffith, J.M., Kleijmeer, M.J., Koster, A.J., Verkleij, A.J., Distel, B., and Tabak, H.F. Mol. Biol. Cell [2003]14, 2900-2907). The traditional view holds that both the peroxisomal matrix and membrane proteins are synthesized on free polyribosomes and delivered to the cytosol, where they are picked up by soluble receptors and guided to the peroxisome. It is not known where the peroxisomal membrane itself comes from. Almost 30 years ago it was already suggested that the ER is associated with so-called microperoxisomes and might, therefore, be a source for the peroxisomal membrane (Novikoff, P.M. and Novikoff, A.B. J. Cell Biol. [1972]53, 532-560). Later peroxisomes were considered autonomous organelles, multiplying by growth and division (Lazarow, P.B., Fujiki, Y. Ann. Rev. Cell Biol. [1985]1, 489-530). The study by Geuze et al. (in this issue) favors a model in which certain membrane proteins are recruited to specialized ER subdomains and argues in favor of the view that the ER is involved in peroxisome formation.

-Hans J. Geuze

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Copyright © 2003 by The American Society for Cell Biology.