PROTEIN-TRANSPORT INTO HIGHER-PLANT PEROXISOMES - IN-VITRO IMPORT ASSAY PROVIDES EVIDENCE FOR RECEPTOR INVOLVEMENT

Peroxisome biogenesis requires that proteins be transported from their site of synthesis in the cytoplasm to their final location in the per oxisome matrix or membrane. Glyoxysomes are a class of peroxisomes fou nd primarily in germinating seedlings and are involved in mobilizing f atty acids via the glyoxylate cycle and the beta-oxidation pathway. We have used an in vitro assay to study the mechanism(s) of import of pr oteins into glyoxysomes. Results from this assay indicate that the tra nsport process is time- and temperature-dependent and is specific for peroxisomal proteins. Isocitrate lyase, a glyoxysomal protein, and the leaf-type peroxisomal enzyme glycolate oxidase (GLO) were transported into pumpkin (Cucurbita pepo) glyoxysomes with no apparent difference s in efficiency of import. Thus, this in vitro assay appears to be phy siologically relevant and correlates well with expected in vivo condit ions. Protein import was also energy-dependent and saturable. Nonradio labeled GLO competed with radiolabeled, in vitro-synthesized GLO for c omponents of the import machinery. Finally, pretreatment of the isolat ed glyoxysomes with protease virtually abolished subsequent import of GLO. Taken together, these results indicate that a proteinaceous recep tor is involved in the import of peroxisomal proteins.