Peroxisomal integral membrane proteins in control and Zellweger fibroblasts

An entire organelle, the peroxisome, appears to be missing in Zellweger syndrome, causing profound neurological problems and neonatal death. One hypothesis for the molecular cause of this defect is a failure in the assembly of the peroxisomal membrane. An alternative is that the peroxisomal membrane is assembled, but the post-translational import of the matrix proteins is defective. We have investigated these possibilities by analytical cell fractionation, immunoblotting, and immunoelectron microscopy of fibroblasts. We identified four integral membrane proteins that can serve as markers for the human peroxisomal membrane. In Zellweger fibroblasts, peroxisomal membranes were found but they were abnormal; they had an equilibrium density of 1.10 g/cm3 instead of the normal density of 1.17 g/cm3, their diameters were generally 2-4 times greater than normal, and they lacked most content. The existence of these peroxisomal ghosts in Zellweger syndrome fibroblasts supports the hypothesis that the defect in this disease is in the protein import machinery.