Periodically fluctuating protein kinases phosphorylate CLOCK, the putativetarget in the suprachiasmatic nucleus

We studied nuclear protein phosphorylation in the rat suprachiasmatic nucle us (SCN) and found that a nuclear fraction of the SCN contained histone H1 kinase activity that periodically fluctuated with a diurnal rhythm, reachin g a maximum at the midpoint of the light phase and a minimum at the midpoin t of the dark phase. A p13(suc1)-bound fraction from the SCN nuclear fracti on also exhibited diurnally fluctuating histone H1 kinase activity. Using i n situ kinase assay, three histone H1 kinases, p45(PFK), p100(PFK), and p20 0(PFK) (termed periodically fluctuating protein kinases, or PFKs) were foun d in the p13(suc1)-bound fractions. p45(PFK) exhibited the highest level of light/dark cycle phosphorylation activity fluctuation. p45(PFK) highly pho sphorylated the Ser-Pro-rich region of CLOCK, the putative physiological ta rget. These results suggest that PFKs, especially p45(PFK), are involved in circadian clock-related signal transduction and gene expression, through t he phosphorylation of target proteins such as CLOCK.