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Molecular and Cellular Biology, June 2002, p. 4346-4357, Vol. 22, No. 12
0270-7306/02/$04.00+0     DOI: 10.1128/MCB.22.12.4346-4357.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

UBA1 and UBA2, Two Proteins That Interact with UBP1, a Multifunctional Effector of Pre-mRNA Maturation in Plants

Mark H. L. Lambermon,^, Yu Fu,^, Dominika A. Wieczorek Kirk,^, Marcel Dupasquier,^,|| Witold Filipowicz,^* Zdravko J. Lorkovi,^,#

Friedrich Miescher Institute for Biomedical Research, 4002 Basel, Switzerland

Received 12 October 2001/ Returned for modification 20 November 2001/ Accepted 21 February 2002

Nicotiana plumbaginifolia UBP1 is an hnRNP-like protein associated with the poly(A)⁺ RNA in the cell nucleus. Consistent with a role in pre-mRNA processing, overexpression of UBP1 in N. plumabaginifolia protoplasts enhances the splicing of suboptimal introns and increases the steady-state levels of reporter mRNAs, even intronless ones. The latter effect of UBP1 is promoter specific and appears to be due to UBP1 binding to the 3' untranslated region (3'-UTR) and protecting the mRNA from exonucleolytic degradation (M. H. L. Lambermon, G. G. Simpson, D. A. Kirk, M. Hemmings-Mieszczak, U. Klahre, and W. Filipowicz, EMBO J. 19:1638-1649, 2000). To gain more insight into UBP1 function in pre-mRNA maturation, we characterized proteins interacting with N. plumbaginifolia UBP1 and one of its Arabidopsis thaliana counterparts, AtUBP1b, by using yeast two-hybrid screens and in vitro pull-down assays. Two proteins, UBP1-associated proteins 1a and 2a (UBA1a and UBA2a, respectively), were identified in A. thaliana. They are members of two novel families of plant-specific proteins containing RNA recognition motif-type RNA-binding domains. UBA1a and UBA2a are nuclear proteins, and their recombinant forms bind RNA with a specificity for oligouridylates in vitro. As with UBP1, transient overexpression of UBA1a in protoplasts increases the steady-state levels of reporter mRNAs in a promoter-dependent manner. Similarly, overexpression of UBA2a increases the levels of reporter mRNAs, but this effect is promoter independent. Unlike UBP1, neither UBA1a nor UBA2a stimulates pre-mRNA splicing. These and other data suggest that UBP1, UBA1a, and UBA2a may act as components of a complex recognizing U-rich sequences in plant 3'-UTRs and contributing to the stabilization of mRNAs in the nucleus.

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* Corresponding author. Mailing address: Friedrich Miescher Institut, Maulbeerstrasse 66, CH-4058 Basel, Switzerland. Phone: 41 61 697 6993. Fax: 41 61 697 3976. E-mail: Filipowi{at}fmi.ch.

Present address: CRND, University of Toronto, Toronto, Ontario M5S 3H2, Canada.

Present address: Department of Microbiology, University of Saskatchewan, Saskatoon, Saskatchewan S7N 5E5, Canada.

Present address: Institut für Pflanzenwissenschaften, ETH Zürich, 8006 Zürich, Switzerland.

^|| Present address: Department of Immunology, Erasmus University Rotterdam, Academic Hospital Dijkzigt, Rotterdam, The Netherlands.

^# Present address: Institut of Medical Biochemistry, Vienna Biocenter, A-1030 Vienna, Austria.

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Molecular and Cellular Biology, June 2002, p. 4346-4357, Vol. 22, No. 12
0022-538X/02/$04.00+0     DOI: 10.1128/MCB.22.12.4346-4357.2002
Copyright © 2002, American Society for Microbiology. All Rights Reserved.

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