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 t o promote mRNA export by acting as adaptors between mRNA and the transport machinery. The existence of multiple shuttling hnRNP proteins raises the qu estion of whether differentially regulated, hnRNP-specific mRNA export path ways exist. Results: We have determined that Tom1p, a conserved protein with a hect (ho mology to EG-AP carboxyl terminus) E3 ubiquitin ligase domain, is required for efficient mRNA export in S. cerevisiae, yet differentially affects hnRN P protein localization and export. Mutations in tom1 predicted to abolish u biquitin ligase activity block efficient export of Nab2p and mRNA, causing Nab2p-mRNA complexes to accumulate in a punctate pattern coincident with th e nuclear pore complex (NPC). Notably, the subcellular distribution of seve ral other hnRNP proteins is not affected. In particular, Npl3p remains mRNA -associated and continues to be efficiently exported in tom1 mutants. Conclusion: Our results demonstrate that mutations predicted to affect the enzymatic activity of the Tom1p ubiquitin ligase differentially affect expo rt of hnRNP proteins in association with mRNA. We propose the existence of multiple mRNA export pathways, with export of Nab2p-associated mRNAs depend ent on a branch of the ubiquitin protein modification pathway. (C) 2000 Els evier Science Ltd. All rights reserved.