Discovery of MARK2 as a physiological kinase for PER2 in the mammalian clock.

Genetics has been a powerful approach in studying the circadian clock, uncovering the first gene Period (Per) as a key regulator. Human mutation in serine 662 (S662) was found in 2001 to cause familial advanced sleep phase (FASP) syndrome with phase advancement and period shortening. We found S662 phosphorylation by casein kinase 1 (CK1) δ and ε, testis-specific serine kinase (TSSK) 1 and 2, and salt inducible kinase (SIK) 1-3, but no phase advancement phenotype after genetic deletion of any of these seven genes. Our biochemical purification revealed microtubule affinity regulating kinase 2 (MARK2) in phosphorylating S662, binding to and stabilizing PER2. Circadian period was shortened in Mark2-deficient cells in an S662-dependent manner. Neuronal specific Mark2 knockout mice showed phase advancement and period shortening. We have discovered MARK2 as a physiologically significant regulator of the clock, and shown the effectiveness of biochemical purification in mechanistic studies of behaviors.