title: BMAL1 shuttling controls transactivation and degradation of the CLOCK/BMAL1 heterodimer.
abstract: 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.
authors.affiliation: School of Biological Sciences, Seoul National University, Seoul 151-742, South Korea.
authors.forename: Ilmin
authors.initials: I
authors.lastname: Kwon
authors.forename: Jiwon
authors.initials: J
authors.lastname: Lee
authors.forename: Seok Hoon
authors.initials: SH
authors.lastname: Chang
authors.forename: Neon Cheol
authors.initials: NC
authors.lastname: Jung
authors.forename: Byung Ju
authors.initials: BJ
authors.lastname: Lee
authors.forename: Gi Hoon
authors.initials: GH
authors.lastname: Son
authors.forename: Kyungjin
authors.initials: K
authors.lastname: Kim
authors.forename: Kun Ho
authors.initials: KH
authors.lastname: Lee
mesh_terms: 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
chemical_list: 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