In brief

Peroxisomal deficiency diseases such as Zellweger syndrome would seem to be a promising place to start in defining how peroxisomes are made. But the cells of all Zellweger patients examined to date have contained peroxisomes. The disease defects appear to be mainly in the import of peroxisomal matrix proteins, an interesting process but not very helpful in determining how an organelle with no genome propagates itself. In this issue, South and Gould (page 255) find a Zellweger cell line that is mutant for the PEX16 gene and lacks detectable peroxisomal structures. Restoration of PEX16 expression causes peroxisome regeneration. How regeneration might work is mysterious. The Zellweger cells contain no membranous organelles with detectable amounts of any 1 of 11 different peroxisomal membrane proteins, so a substrate for Pex16 action, a preperoxisomal vesicle, remains a hypothetical entity. Peroxisome regeneration is slow, suggesting that most peroxisome proliferation in normal cells is directed by other means, such as Pex11 a/b –mediated vesiculation of existing peroxisomes. This pathway still requires membrane insertion of newly synthesized integral membrane proteins into peroxisomes, which may be the real (if unproven) function of Pex16.