PubMed · 8791461
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I Mellman. 1996. Membranes and sorting.. https://doi.org/10.1016/s0955-0674(96)80026-3
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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.
The subcellular localization of the TNF receptor-associated factor-2 (TRAF2) adaptor protein in human endothelial cells, which mediates proinflammatory responses of TNF, has been analyzed by confocal immunofluorescence microscopy and by Western blotting of fractionated cell extracts. Rabbit antisera reactive with either amino- or carboxyl-terminal TRAF2 peptides frequently but not uniformly stain nuclei of cultured HUVEC or the established human endothelial cell line, ECV304. However, Western blotting reveals significant heterogeneity in the reactivities of these polyclonal Abs. Transiently transfected HUVEC expressing FLAG epitope-tagged TRAF2 consistently show prominent nuclear localization, and deletion mutants of TRAF2 identify the portion of the molecule responsible for nuclear localization as the amino-terminal ring finger domain. TNF treatment does not appear to influence the localization of endogenous or transfected TRAF2 protein. Transfection of the amino-terminal half of the TRAF2 molecule, containing the ring and zinc finger domains, which localizes to the nucleus, results in activation of E-selectin but not of NF-kappaB promoter-reporter gene transcription or of c-Jun N-terminal kinase activation. These observations suggest that TRAF2 may reside in the nucleus and directly regulate transcription, independent of its role in cytoplasmic signal transduction.
Yeast cells mutated in YRB2, which encodes a nuclear protein with similarity to other Ran-binding proteins, fail to export nuclear export signal (NES)-containing proteins including HIV Rev out of the nucleus. Unlike Xpo1p/Crm1p/exportin, an NES receptor, Yrb2p does not shuttle between the nucleus and the cytoplasm but instead remains inside the nucleus. However, by both biochemical and genetic criteria, Yrb2p interacts with Xpo1p and not with other members of the importin/karyopherin beta superfamily. Moreover, the Yrb2p region containing nucleoporin-like FG repeats is important for NES-mediated protein export. Taken together, these data suggest that Yrb2p acts inside the nucleus to mediate the action of Xpo1p in at least one of several nuclear export pathways.