A putative ubiquitin ligase required for efficient mRNA export differentially affects hnRNP transport.
BACKGROUND
In the nucleus, mRNAs are bound by hnRNP proteins. A subset of hnRNP proteins shuttle between the nucleus and cytoplasm and are believed to promote mRNA export by acting as adaptors between mRNA and the transport machinery. The existence of multiple shuttling hnRNP proteins raises the question of whether differentially regulated, hnRNP-specific mRNA export pathways exist.


RESULTS
We have determined that Tom1p, a conserved protein with a hect (homology to E6-AP carboxyl terminus) E3 ubiquitin ligase domain, is required for efficient mRNA export in S. cerevisiae, yet differentially affects hnRNP protein localization and export. Mutations in tom1 predicted to abolish ubiquitin ligase activity block efficient export of Nab2p and mRNA, causing Nab2p-mRNA complexes to accumulate in a punctate pattern coincident with the nuclear pore complex (NPC). Notably, the subcellular distribution of several other hnRNP proteins is not affected. In particular, Np13p remains mRNA-associated and continues to be efficiently exported in tom1 mutants.


CONCLUSIONS
Our results demonstrate that mutations predicted to affect the enzymatic activity of the Tom1p ubiquitin ligase differentially affect export of hnRNP proteins in association with mRNA. We propose the existence of multiple mRNA export pathways, with export of Nab2p-associated mRNAs dependent on a branch of the ubiquitin protein modification pathway.
Duncan
K
K
Department of Biochemistry and Biophysics, University of California, San Francisco 94143-0448, USA.
Umen
J G
JG
Guthrie
C
C
D000595:Amino Acid Sequence; D001692:Biological Transport; D003001:Cloning, Molecular; D017124:Conserved Sequence; D019143:Evolution, Molecular; D005656:Fungal Proteins / Q000378:metabolism; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D018398:Homeodomain Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008025:Ligases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008969:Molecular Sequence Data; D009154:Mutation; D009687:Nuclear Proteins / Q000378:metabolism; D029543:Nucleocytoplasmic Transport Proteins; D012333:RNA, Messenger / Q000378:metabolism*; D016601:RNA-Binding Proteins / Q000378:metabolism; D012261:Ribonucleoproteins / Q000378:metabolism*; D029701:Saccharomyces cerevisiae Proteins; D017386:Sequence Homology, Amino Acid; D044767:Ubiquitin-Protein Ligases; D014452:Ubiquitins / Q000378:metabolism*
D005656:Fungal Proteins; D034441:Heterogeneous-Nuclear Ribonucleoproteins; D018398:Homeodomain Proteins; C080773:NAB2 protein, S cerevisiae; C077734:NPL3 protein, S cerevisiae; D009687:Nuclear Proteins; D029543:Nucleocytoplasmic Transport Proteins; D012333:RNA, Messenger; D016601:RNA-Binding Proteins; D012261:Ribonucleoproteins; D029701:Saccharomyces cerevisiae Proteins; D014452:Ubiquitins; C115246:TOM1 protein, S cerevisiae; D044767:Ubiquitin-Protein Ligases; D008025:Ligases