The 22-kD peroxisomal integral membrane protein in zellweger syndrome—presence, abundance, and association with a peroxisomal thiolase precursor protein

The primary genetic defect of Zellweger syndrome may be related to defective synthesis or impaired import of peroxisomal proteins. We analyzed the presence and measured the abundance of the 22-kD peroxisomal integral membrane protein (PMP) in patients with Zellweger syndrome. We determined the intracellular localization of the 22-kD PMP and compared it with the localization of a peroxisomal 44-kD thiolase precursor protein. The 22-kD PMP was quantified by immunoblot analyses in liver tissue (n = 7 patients). Immunoblot signals were evaluated using transmission photometry. The intracellular localization of the 22-kD PMP and the peroxisomal 44-kD thiolase precursor protein were determined by immu-noblot analyses on fibroblast subcellular fractions prepared by Nycodenz (n = 5 patients) or sucrose density gradient centrifugation (n = 2 patients). The 22-kD PMP was present and associated with membrane fractions in all patients. Its abundance varied in patients as compared with normal human liver controls. The 22-kD PMP was located in subcellular membrane fractions having a lower density than normal peroxisomes or mitochondria. Using two different gradient techniques, the 22-kD PMP and the peroxisomal 44-kD thiolase precursor protein were found in the same low-density gradient fractions. These results suggest that in Zellweger syndrome peroxisome-like elements containing both the 22-kD PMP and a 44-kD thiolase precursor protein are formed. Globally defective synthesis or import of peroxisomal proteins is therefore unlikely to be the primary genetic defect in the patients we studied.