The effects of chaperones and the influence of protein assembly on peroxisomal protein import.

Peroxisomal proteins are synthesized in the cytoplasm and post-translationally translocated into the organelle. The role of chaperones and protein folding in peroxisomal protein transport is still unclear. Translocation of proteins into mitochondria requires that precursor proteins assume an extended conformation; cytosolic chaperones are thought to help maintain this conformation. In contrast, peroxisomal protein import does not require unfolding of the targeted protein. However, the molecular chaperones Hsp70 and Hsp40 may be important for translocation. We present several lines of evidence that show that plant peroxisomal protein import is enhanced by chaperones. First, peroxisomes isolated from heat-shocked pumpkin seedling tissues exhibited increased protein import relative to control peroxisomes. Second, antibodies raised against wheat germ cytosolic Hsp70 and Escherichia coli Hsp90 inhibited import of the peroxisomal protein isocitrate lyase. To our knowledge, this is the first time that Hsp90 has been directly implicated in a protein transport event. Third, peroxisomal proteins were immunoprecipitated by wheat germ Hsp70 antibodies. We also present results that suggest that the efficiency of peroxisomal protein import is influenced by the structure of the targeted protein; monomeric isocitrate lyase was imported more efficiently than oligomeric isocitrate lyase. Taken together, these data demonstrate that the assembly state of peroxisomal proteins and the chaperones that may mediate those states are both important for efficient peroxisomal protein import.