Stem-Loop Binding Protein, the Protein That Binds the 3′ End of Histone mRNA, Is Cell Cycle Regulated by Both Translational and Posttranslational Mechanisms

Abstract

ABSTRACT The expression of the replication-dependent histone mRNAs is tightly regulated during the cell cycle. As cells progress from G 1 to S phase, histone mRNA levels increase 35-fold, and they decrease again during G 2 phase. Replication-dependent histone mRNAs are the only metazoan mRNAs that lack polyadenylated tails, ending instead in a conserved stem-loop. Much of the cell cycle regulation is posttranscriptional and is mediated by the 3′ stem-loop. A 31-kDa stem-loop binding protein (SLBP) binds the 3′ end of histone mRNA. The SLBP is necessary for pre-mRNA processing and accompanies the histone mRNA to the cytoplasm, where it is a component of the histone messenger RNP. We used synchronous CHO cells selected by mitotic shakeoff and HeLa cells synchronized at the G 1 /S or the M/G 1 boundary to study the regulation of SLBP during the cell cycle. In each system the amount of SLBP is regulated during the cell cycle, increasing 10- to 20-fold in the late G 1 and then decreasing in the S/G 2 border. SLBP mRNA levels are constant during the cell cycle. SLBP is regulated at the level of translation as cells progress from G 1 to S phase, and the protein is rapidly degraded as they progress into G 2 . Regulation of SLBP may account for the posttranscriptional component of the cell cycle regulation of histone mRNA.