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

Abstract

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.

We thank Jirina Petruska for technical assistance. We are grateful to Andrea Barta and Dorothee Staiger for providing chemicals for experiments performed during revision of the manuscript. The Arabidopsis Biological Resource Center, The Ohio State University, is acknowledged for providing A. thaliana EST clones.

Z.J.L. was the recipient of a long-term fellowship from the European Molecular Biology Organization (EMBO). The Friedrich Miescher Institute for Biomedical Research is part of the Novartis Research Foundation.