Inhibitors of COPI and COPII do not block PEX3-mediated peroxisome synthesis

In humans, defects in peroxisome biogenesis are the cause of lethal disease s typified by Zellweger syndrome. Here, we show that inactivating mutations in human PEX3 cause Zellweger syndrome, abrogate peroxisome membrane synth esis, and result in reduced abundance of peroxisomal membrane proteins (PMP s) and/or mislocalization of PMPs to the mitochondria. Previous studies hav e suggested that PEX3 may traffic through the ER en route to the peroxisome , that the COPI inhibitor, brefeldin A, leads to accumulation of PEX3 in th e ER, and that PEX3 overexpression alters the morphology of the ER. However , we were unable to detect PEX3 in the ER at early times after expression. Furthermore, we find that inhibition of COPI function by brefeldin A has no effect on trafficking of PEX3 to peroxisomes and does not inhibit PEX3-med iated peroxisome biogenesis. We also find that inhibition of COPII-dependen t membrane traffic by a dominant negative SAR1 mutant fails to block PEX3 t ransport to peroxisomes and PEX3-mediated peroxisome synthesis. Based on th ese results, we propose that PEX3 targeting to peroxisomes and PEX3-mediate d peroxisome membrane synthesis may occur independently of COPI- and COPII- dependent membrane traffic.