BMAL1 shuttling controls transactivation and degradation of the CLOCK/BMAL1 heterodimer.
CLOCK and BMAL1 are bHLH-PAS-containing transcription factors that bind to E-box elements and are indispensable for expression of core circadian clock components such as the Per and Cry genes. A key step in expression is the heterodimerization of CLOCK and BMAL1 and their accumulation in the nucleus with an approximately 24-h periodicity. We show here that nucleocytoplasmic shuttling of BMAL1 is essential for transactivation and for degradation of the CLOCK/BMAL1 heterodimer. Using serial deletions and point mutants, we identified a functional nuclear localization signal and Crm1-dependent nuclear export signals in BMAL1. Transient-transfection experiments revealed that heterodimerization of CLOCK and BMAL1 accelerates their turnover, as well as E-box-dependent clock gene transcription. Moreover, in embryonic mouse fibroblasts, robust transcription of Per2 is tightly associated with massive degradation of the CLOCK/BMAL1 heterodimer. CRY proteins suppressed this process during the transcription-negative phase and led to nuclear accumulation of the CLOCK/BMAL1 heterodimer. Thus, these findings suggest that the decrease of BMAL1 abundance during the circadian cycle reflects robust transcriptional activation of clock genes rather than inhibition of BMAL1 synthesis.
Kwon
Ilmin
I
School of Biological Sciences, Seoul National University, Seoul 151-742, South Korea.
Lee
Jiwon
J
Chang
Seok Hoon
SH
Jung
Neon Cheol
NC
Lee
Byung Ju
BJ
Son
Gi Hoon
GH
Kim
Kyungjin
K
Lee
Kun Ho
KH
D056930:ARNTL Transcription Factors; D000595:Amino Acid Sequence; D000818:Animals; D051792:Basic Helix-Loop-Helix Proteins / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D056926:CLOCK Proteins; D002467:Cell Nucleus / Q000378:metabolism; D002478:Cells, Cultured; D002940:Circadian Rhythm / Q000502:physiology; D003593:Cytoplasm / Q000378:metabolism; D019281:Dimerization; D006367:HeLa Cells; D006801:Humans; D051379:Mice; D008954:Models, Biological; D008969:Molecular Sequence Data; D050505:Mutant Proteins / Q000378:metabolism; D041681:NIH 3T3 Cells; D049790:Nuclear Export Signals; D019913:Nuclear Localization Signals / Q000737:chemistry; D011499:Protein Processing, Post-Translational; D017434:Protein Structure, Tertiary; D021381:Protein Transport; D015534:Trans-Activators / Q000378:metabolism*; D014158:Transcription, Genetic / Q000235:genetics; D015533:Transcriptional Activation; D025801:Ubiquitin / Q000378:metabolism
D056930:ARNTL Transcription Factors; D051792:Basic Helix-Loop-Helix Proteins; D056926:CLOCK Proteins; D050505:Mutant Proteins; D049790:Nuclear Export Signals; D019913:Nuclear Localization Signals; D015534:Trans-Activators; D025801:Ubiquitin; C538960:BMAL1 protein, human; C538961:Bmal1 protein, mouse; C538943:CLOCK protein, human; C538944:Clock protein, mouse