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Molecular and Cellular Biology, March 2003, p. 1590-1601, Vol. 23, No. 5
0270-7306/03/$08.00+0     DOI: 10.1128/MCB.23.5.1590-1601.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.

Phosphorylation of Stem-Loop Binding Protein (SLBP) on Two Threonines Triggers Degradation of SLBP, the Sole Cell Cycle-Regulated Factor Required for Regulation of Histone mRNA Processing, at the End of S Phase

Lianxing Zheng,1,2 Zbigniew Dominski,1,2 Xiao-Cui Yang,2 Phillip Elms,1 Christy S. Raska,1 Christoph H. Borchers,1 and William F. Marzluff1,2*

Department of Biochemistry and Biophysics,1 Program in Molecular Biology and Biotechnology, University of North Carolina, Chapel Hill, North Carolina2

Received 2 October 2002/ Returned for modification 14 November 2002/ Accepted 9 December 2002

The replication-dependent histone mRNAs, the only eukaryotic mRNAs that do not have poly(A) tails, are present only in S-phase cells. Coordinate posttranscriptional regulation of histone mRNAs is mediated by the stem-loop at the 3' end of histone mRNAs. The protein that binds the 3' end of histone mRNA, stem-loop binding protein (SLBP), is required for histone pre-mRNA processing and is involved in multiple aspects of histone mRNA metabolism. SLBP is also regulated during the cell cycle, accumulating as cells enter S phase and being rapidly degraded as cells exit S phase. Mutation of any residues in a TTP sequence (amino acids 60 to 62) or mutation of a consensus cyclin binding site (amino acids 99 to 104) stabilizes SLBP in G2 and mitosis. These two threonines are phosphorylated in late S phase, as determined by mass spectrometry (MS) of purified SLBP from late S-phase cells, triggering SLBP degradation. Cells that express a stable SLBP still degrade histone mRNA at the end of S phase, demonstrating that degradation of SLBP is not required for histone mRNA degradation. Nuclear extracts from G1 and G2 cells are deficient in histone pre-mRNA processing, which is restored by addition of recombinant SLBP, indicating that SLBP is the only cell cycle-regulated factor required for histone pre-mRNA processing.

* Corresponding author. Mailing address: Program in Molecular Biology and Biotechnology, CB 7100, University of North Carolina, Chapel Hill, NC 27599. Phone: (919) 962-8920. Fax: (919) 966-6821. E-mail: marzluff{at}med.unc.edu.

Molecular and Cellular Biology, March 2003, p. 1590-1601, Vol. 23, No. 5
0022-538X/03/$08.00+0     DOI: 10.1128/MCB.23.5.1590-1601.2003
Copyright © 2003, American Society for Microbiology. All Rights Reserved.



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