Mammalian CLOCK and BMAL1 are two members of bHLH-PAS-containing family of 
transcription factors that represent the positive elements of circadian 
autoregulatory feedback loop. In the form of a heterodimer, they drive 
transcription from E-box enhancer elements in the promoters of responsive genes. 
We have examined abundance, posttranslational modifications, cellular 
localization of endogenous and ectopically expressed CLOCK and BMAL1 proteins. 
Nuclear/cytoplasm distribution of CLOCK was found to be under circadian 
regulation. Analysis of subcellular localization of CLOCK in embryo fibroblasts 
of mice carrying different germ-line circadian mutations showed that circadian 
regulation of nuclear accumulation of CLOCK is BMAL1-dependent. Formation of 
CLOCK/BMAL1 complex following ectopic coexpression of both proteins is followed 
by their codependent phosphorylation, which is tightly coupled to CLOCK nuclear 
translocation and degradation. This binding-dependent coregulation is specific 
for CLOCK/BMAL1 interaction, as no other PAS domain protein that can form a 
complex with either CLOCK or BMAL1 was able to induce similar effects. 
Importantly, all posttranslational events described in our study are coupled 
with active transactivation complex formation, which argues for their 
significant functional role. Altogether, these results provide evidence for an 
additional level of circadian system control, which is based on regulation of 
transcriptional activity or/and availability of CLOCK/BMAL1 complex.