Import of stably-folded proteins into peroxisomes.

By virtue of their synthesis in the cytoplasm, proteins destined for import into peroxisomes are obliged to traverse the single membrane of this organelle. Because the targeting signal for most peroxisomal matrix proteins is a carboxy-terminal tripeptide sequence (SKL or its variants), these proteins must remain import-competent until their translation is complete. Although the conformational requirements for translocation across other cellular membranes are known in some detail, they are presently unknown for the peroxisomal membrane. Prefolded proteins stabilized with disulfide bonds and chemical cross-linkers were shown to be substrates for peroxisomal import, as were mature folded and disulfide-bonded IgG molecules containing the peroxisomal targeting signal. In addition, colloidal gold particles conjugated to proteins bearing the peroxisomal targeting signal were translocated into the peroxisomal matrix. These results support the concept that proteins may fold in the cytosol prior to their import into the peroxisome, and that protein unfolding is not a prerequisite for peroxisomal import.