Phosphorylation of threonine 61 by cyclin a/Cdk1 triggers degradation of stem-loop binding protein at the end of S phase.
Histone mRNA levels are cell cycle regulated, and a major regulatory mechanism is restriction of stem-loop binding protein (SLBP) to S phase. Degradation of SLBP at the end of S phase results in cessation of histone mRNA biosynthesis, preventing accumulation of histone mRNA until SLBP is synthesized just before entry into the next S phase. Degradation of SLBP requires an SFTTP (58 to 62) and KRKL (95 to 98) sequence, which is a putative cyclin binding site. A fusion protein with the 58-amino-acid sequence of SLBP (amino acids 51 to 108) fused to glutathione S-transferase (GST) is sufficient to mimic SLBP degradation at late S phase. Using GST-SLBP fusion proteins as a substrate, we show that cyclin A/Cdk1 phosphorylates Thr61. Furthermore, knockdown of Cdk1 by RNA interference stabilizes SLBP at the end of S phase. Phosphorylation of Thr61 is necessary for subsequent phosphorylation of Thr60 by CK2 in vitro. Inhibitors of CK2 also prevent degradation of SLBP at the end of S phase. Thus, phosphorylation of Thr61 by cyclin A/Cdk1 primes phosphorylation of Thr60 by CK2 and is responsible for initiating SLBP degradation. We conclude that the increase in cyclin A/Cdk1 activity at the end of S phase triggers degradation of SLBP at S/G(2).
Koseoglu
M Murat
MM
Department of Biology, Program in Molecular Biology and Biotechnology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA.
Graves
Lee M
LM
Marzluff
William F
WF
D016203:CDC2 Protein Kinase / Q000235:genetics / Q000378:metabolism*; D047390:Casein Kinase II / Q000378:metabolism; D019925:Cyclin A / Q000378:metabolism*; D006367:HeLa Cells; D006801:Humans; D009687:Nuclear Proteins / Q000378:metabolism*; D010766:Phosphorylation; D034622:RNA Interference; D016196:S Phase; D013912:Threonine / Q000378:metabolism; D039221:mRNA Cleavage and Polyadenylation Factors / Q000378:metabolism*
D019925:Cyclin A; D009687:Nuclear Proteins; C092663:SLBP protein, human; D039221:mRNA Cleavage and Polyadenylation Factors; D013912:Threonine; D047390:Casein Kinase II; D016203:CDC2 Protein Kinase