Import of Stably Folded Proteins into Peroxisomes

Abstract

Virtually all proteins destined for the peroxisomal matrix and membrane are synthesized on free polysomes in the cytoplasm, and are imported into the peroxisome post-translationally (Lazarow and Fujiki, 1985; Subramani, 1994). Although two peroxisomal proteins, thiolase, and sterol carrier protein 2, undergo proteolytic cleavage after import, most proteins are synthesized at their mature size. This differs from the mechanism of post-translational import of proteins into the chloroplast, and mitochondria, where the targeting signal resides as an amino-terminal leader sequence that is cleaved following import. Import of proteins into the matrix of the peroxisome is dependent upon the presence of a peroxisomal targeting signal in the amino acid sequence of the newly synthesized protein. Two forms of this peroxisomal targeting signal have been characterized: a C-terminal tripeptide with the sequence serine-lysine leucine or a conservative variant thereof which has been found in many peroxisomal proteins (Gould et al., 1989), or an amino-terminal leader sequence such as that found in peroxisomal thiolase (Swinkels et al., 1991).