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Nucleic Acids Research Advance Access published online on March 29, 2008
Nucleic Acids Research, doi:10.1093/nar/gkn144
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© 2008 The Author(s)
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.

Molecular Biology

A BBP–Mud2p heterodimer mediates branchpoint recognition and influences splicing substrate abundance in budding yeast

Qiang Wang1, Li Zhang1, Bert Lynn2 and Brian C. Rymond1,*

1Department of Biology and 2Department of Chemistry, University of Kentucky, Lexington, KY 40506-0225, USA

*To whom correspondence should be addressed. Tel: 859 257 5530; Fax: 859 257 1717; Email: rymond{at}uky.edu

Received November 30, 2007. Revised February 27, 2008. Accepted March 14, 2008.

The 3' end of mammalian introns is marked by the branchpoint binding protein, SF1, and the U2AF65-U2AF35 heterodimer bound at an adjacent sequence. Baker's yeast has equivalent proteins, branchpoint binding protein (BBP) (SF1) and Mud2p (U2AF65), but lacks an obvious U2AF35 homolog, leaving open the question of whether another protein substitutes during spliceosome assembly. Gel filtration, affinity selection and mass spectrometry were used to show that rather than a U2AF65/U2AF35-like heterodimer, Mud2p forms a complex with BBP without a third (U2AF35-like) factor. Using mutants of MUD2 and BBP, we show that the BBP–Mud2p complex bridges partner-specific Prp39p, Mer1p, Clf1p and Smy2p two-hybrid interactions. In addition to inhibiting Mud2p association, the bbp{Delta}56 mutation impairs splicing, enhances pre-mRNA release from the nucleus, and similar to a mud2::KAN knockout, suppresses a lethal sub2::KAN mutation. Unexpectedly, rather than exacerbating bbp{Delta}56, the mud2::KAN mutation partially suppresses a pre-mRNA accumulation defect observed with bbp{Delta}56. We propose that a BBP–Mud2p heterodimer binds as a unit to the branchpoint in vivo and serves as a target for the Sub2p-DExD/H-box ATPase and for other splicing factors during spliceosome assembly. In addition, our results suggest the possibility that the Mud2p may enhance the turnover of pre-mRNA with impaired BBP-branchpoint association.

Present address: Qiang Wang, Department of Medicine, Division of Biological Sciences, The University of Chicago, 5841 S. Maryland Ave., Chicago, IL 60637, USA


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