Nuclear export of the oncoprotein v-ErbA is mediated by acquisition of a viral nuclear export sequence.
v-ErbA, an oncogenic derivative of the thyroid hormone receptor alpha (TRalpha) carried by the avian erythroblastosis virus, contains several alterations including fusion of a portion of avian erythroblastosis virus Gag to its N terminus, N- and C-terminal deletions, and 13 amino acid substitutions. Nuclear export of v-ErbA occurs through a CRM1-mediated pathway. In contrast, nuclear export of TRalpha and another isoform, TRbeta, is CRM1-independent. To determine which amino acid changes in v-ErbA confer CRM1-dependent nuclear export, we expressed a panel of green and yellow fluorescent protein-tagged mutant and chimeric proteins in mammalian cells. The sensitivity of subcellular trafficking of these mutants to leptomycin B (LMB), a specific inhibitor of CRM1, was assessed by fluorescence microscopy. Our data showed that a nuclear export sequence resides within a 70-amino acid domain in the C-terminal portion of the p10 region of Gag, and in vitro binding assays demonstrated that Gag interacts directly with CRM1. However, a panel of ligand-binding domain mutants of v-ErbA lacking the Gag sequence exhibited greater nuclear localization in the presence of LMB, suggesting that the various amino acid substitutions/deletions may cause a conformation shift, unmasking an additional CRM1-dependent nuclear export sequence. In contrast, the altered DNA-binding domain of the oncoprotein did not contribute to CRM1-dependent nuclear export. Heterokaryon experiments revealed that v-ErbA did not undergo nucleocytoplasmic shuttling when the CRM1 export pathway was blocked by LMB treatment, suggesting that the ability to follow the export pathway used by TRalpha has been lost by the oncoprotein during its evolution. Our findings thus point to the intriguing possibility that acquisition of altered nuclear export capabilities contributes to the oncogenic properties of v-ErbA.
DeLong
Laura J
LJ
Department of Biology, College of William and Mary, Williamsburg, Virginia 23187, USA.
Bonamy
Ghislain M C
GM
Fink
Erin N
EN
Allison
Lizabeth A
LA
D021581:Active Transport, Cell Nucleus; D000595:Amino Acid Sequence; D000818:Animals; D002467:Cell Nucleus / Q000378:metabolism; D003593:Cytoplasm / Q000378:metabolism; D005231:Fatty Acids, Unsaturated / Q000737:chemistry; D017353:Gene Deletion; D049452:Green Fluorescent Proteins; D006367:HeLa Cells; D006801:Humans; D028884:Karyopherins / Q000737:chemistry / Q000378:metabolism / Q000502:physiology*; D008024:Ligands; D008164:Luminescent Proteins / Q000378:metabolism; D051379:Mice; D008856:Microscopy, Fluorescence; D008969:Molecular Sequence Data; D009154:Mutation; D041681:NIH 3T3 Cells; D018774:Oncogene Proteins v-erbA / Q000378:metabolism / Q000502:physiology*; D010957:Plasmids / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D020033:Protein Isoforms; D017434:Protein Structure, Tertiary; D018160:Receptors, Cytoplasmic and Nuclear; D011993:Recombinant Fusion Proteins / Q000378:metabolism; D017386:Sequence Homology, Amino Acid; D013997:Time Factors; D014162:Transfection; D000097606:Exportin 1 Protein
D005231:Fatty Acids, Unsaturated; D049452:Green Fluorescent Proteins; D028884:Karyopherins; D008024:Ligands; D008164:Luminescent Proteins; D018774:Oncogene Proteins v-erbA; D020033:Protein Isoforms; D018160:Receptors, Cytoplasmic and Nuclear; D011993:Recombinant Fusion Proteins; D000097606:Exportin 1 Protein; C038753:leptomycin B