Identification of RACK1 and Protein Kinase Cα as Integral Components of the Mammalian Circadian Clock

Late-Running Clock Components Many mammalian cells contain a well-characterized biological clock with a 24-hour cycle. In the latter part of the day, transcription mediated by one of the clock components, the transcription factor made up of the CLOCK and BMAL1 proteins, is inhibited, but the mechanism of inhibition has been unclear. Robles et al. (p. 463 ) used mass spectrometry to identify proteins that RACK1 (receptor for activated C kinase–1), a scaffold protein that brings protein kinase C–α (PKCα ) into contact with its substrates, caused to be associated with BMAL1 at the time of day when its transcription-activating function was inhibited. Further studies implicated PKCα and RACK1 as integral components of the clock, without which the clock's free-running period was shortened.