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The Journal of Neuroscience, January 14, 2009, 29(2):466-475; doi:10.1523/JNEUROSCI.4034-08.2009

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Cellular/Molecular
Ribosomal S6 Kinase Cooperates with Casein Kinase 2 to Modulate the Drosophila Circadian Molecular Oscillator

Bikem Akten,^1 * Michelle M. Tangredi,^1 * Eike Jauch,² Mary A. Roberts,¹ Fanny Ng,¹ Thomas Raabe,² and F. Rob Jackson¹

¹Department of Neuroscience, Tufts Center for Neuroscience Research, Sackler School of Graduate Biomedical Sciences, Tufts University School of Medicine, Boston, Massachusetts 02111, and ²University of Wuerzburg, Institut für Medizinische Strahlenkunde und Zellforschung, 97078 Wuerzburg, Germany

Correspondence should be addressed to F. Rob Jackson, Department of Neuroscience, Tufts Center for Neuroscience Research, Sackler School of Graduate Biomedical Sciences, Tufts University School of Medicine, 136 Harrison Avenue, Boston, MA 02111. Email: rob.jackson{at}tufts.edu

There is a universal requirement for post-translational regulatory mechanisms in circadian clock systems. Previous work in Drosophila has identified several kinases, phosphatases, and an E3 ligase that are critical for determining the nuclear translocation and/or stability of clock proteins. The present study evaluated the function of p90 ribosomal S6 kinase (RSK) in the Drosophila circadian system. In mammals, RSK1 is a light- and clock-regulated kinase known to be activated by the mitogen-activated protein kinase pathway, but there is no direct evidence that it functions as a component of the circadian system. Here, we show that Drosophila S6KII RNA displays rhythms in abundance, indicative of circadian control. Importantly, an S6KII null mutant exhibits a short-period circadian phenotype that can be rescued by expression of the wild-type gene in clock neurons, indicating a role for S6KII in the molecular oscillator. Peak PER clock protein expression is elevated in the mutant, indicative of enhanced stability, whereas per mRNA level is decreased, consistent with enhanced feedback repression. Gene reporter assays show that decreased S6KII is associated with increased PER repression. Surprisingly, we demonstrate a physical interaction between S6KII and the casein kinase 2 regulatory subunit (CK2β), suggesting a functional relationship between the two kinases. In support of such a relationship, there are genetic interactions between S6KII and CK2 mutations, in vivo, which indicate that CK2 activity is required for S6KII action. We propose that the two kinases cooperate within clock neurons to fine-tune circadian period, improving the precision of the clock mechanism.

Key words: circadian; post-transcriptional; p90 ribosomal S6 kinase; RSK1; casein kinase 2; MAP kinase

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Received June 26, 2008; revised Dec. 8, 2008; accepted Dec. 9, 2008.

Correspondence should be addressed to F. Rob Jackson, Department of Neuroscience, Tufts Center for Neuroscience Research, Sackler School of Graduate Biomedical Sciences, Tufts University School of Medicine, 136 Harrison Avenue, Boston, MA 02111. Email: rob.jackson{at}tufts.edu

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