Synthesis of a major integral membrane polypeptide of rat liver peroxisomes on free polysomes.

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Abstract
The manner of synthesis and assembly of the peroxisomal membrane proteins is unknown. Understanding these processes is essential to an understanding of the formation of the organelle. The biogenesis of the previously identified major 21.7-kDa [kilodalton] integral peroxisomal membrane polypeptide (Fujiki, Y., Fowler, S., Shio, H., Hubbard, A. L. and Lazarow, P. B., 1982) was investigated. This protein was purified to apparent homogeneity and used to elicit a rabbit antiserum. In immunoblotting analysis, antibody bound only to the 22-kDa membrane polypeptide present exclusively in peroxisomal membranes. Total rat liver RNA was translated in a nuclease-treated rabbit reticulocyte cell-free protein-synthesizing system. The in vitro translation product, isolated by means of the antibody and Staphylococcus aureus cells, comigrated with the mature 22-kDa polypeptide in NaDodSO4/PAGE [sodium dodecylsulfate/polyacrylamide gel electrophoresis]. Analysis of the translation products of RNAs from free and membrane-bound polysomes indicated that the mRNA for the 22-kDa membrane polypeptide is found predominantly in free polysomes. The results imply posttranslational insertion of the membrane polypeptide into the peroxisomal membrane without proteolytic processing and suggest that peroxisomes, like mitochondria and chloroplasts, form by fission from preexisting organelles.