Basic helix-loop-helix ARNT-like protein 1 or aryl hydrocarbon receptor nuclear translocator-like protein 1 (ARNTL), or brain and muscle ARNT-like 1 is a protein that in humans is encoded by the BMAL1 gene on chromosome 11, region p15.3. It's also known as MOP3, and, less commonly, bHLHe5, BMAL, BMAL1C, JAP3, PASD3, and TIC.
BMAL1 encodes a transcription factor with a basic helix-loop-helix (bHLH) and two PAS domains. The human BMAL1 gene has a predicted 24 exons, located on the p15 band of the 11th chromosome. The BMAL1 protein is 626 amino acids long and plays a key role as one of the positive elements in the mammalian auto-regulatory transcription-translation negative feedback loop (TTFL), which is responsible for generating molecular circadian rhythms. Research has revealed that BMAL1 is the only clock gene without which the circadian clock fails to function in humans. BMAL1 has also been identified as a candidate gene for susceptibility to hypertension, diabetes, and obesity, and mutations in BMAL1 have been linked to infertility, gluconeogenesis and lipogenesis problems, and altered sleep patterns. BMAL1, according to genome-wide profiling, is estimated to target more than 150 sites in the human genome, including all of the clock genes and genes encoding for proteins that regulate metabolism.
History
The BMAL1 gene was originally discovered in 1997 by two groups of researchers, John B. Hogenesch et al. in March under the name MOP3 and Ikeda and Nomura in April as part of a superfamily of PAS domain transcription factors. In 1998, Hogenesch's additional characterization of MOP3 revealed that its role as the partner of bHLH-PAS transcription factor CLOCK was essential to mammalian circadian clock function. The MOP3 protein, as it was originally known by the Hogenesch group, was found to dimerize with MOP4, CLOCK, and hypoxia-inducible factors. The names BMAL1 and ARNTL were adopted in later papers. One of BMAL1 protein's earliest discovered functions in circadian regulation was related to the CLOCK-BMAL1 (CLOCK-ARNTL) heterodimer, which would bind through an E-box enhancer to activate the transcription of the AVP gene which encodes for vasopressin. However, the gene's importance in circadian rhythms was not fully realized until the knockout of the gene in mice showed complete loss of circadian rhythms in locomotion and other behaviors.
Genetics
Regulation of Bmal1 activity
SIRT1 regulates PER protein degradation by inhibiting transcriptional activity of the BMAL1:CLOCK heterodimer in a circadian manner through deacetylation. The degradation of PER proteins prevents the formation of the large protein complex, and thus disinhibits the transcriptional activity of the BMAL1:CLOCK heterodimer. The CRY protein is also signaled for degradation by poly-ubiquitination from the FBXL3 protein resulting in the disinhibition of BMAL1:CLOCK heterodimer activity.
In addition to the circadian regulatory TTFL loop, Bmal1 transcription is regulated by competitive binding to the retinoic acid-related orphan receptor response element-binding site (RORE) within the promoter of Bmal1. The CLOCK/BMAL1 heterodimer also binds to E-box elements in promoter regions of Rev-Erbα and RORα/ß genes, upregulating transcription and translation of REV-ERB and ROR proteins. REV-ERBα and ROR proteins regulate BMAL1 expression through a secondary feedback loop and compete to bind to Rev-Erb/ROR response elements in the Bmal1 promoter, resulting in BMAL1 expression repressed by REV-ERBα and activated by ROR proteins. Other nuclear receptors of the same families (NR1D2 (Rev-erb-β); NR1F2 (ROR-β); and NR1F3 (ROR-γ)) have also been shown to act on Bmal1 transcriptional activity in a similar manner.
Several posttranslational modifications of BMAL1 dictate the timing of the CLOCK/BMAL1 feedback loops. Phosphorylation of BMAL1 targets it for ubiquitination and degradation, as well as deubiquitination and stabilization. Acetylation of BMAL1 recruits CRY1 to suppress the transactivation of CLOCK/BMAL1. The sumoylation of BMAL1 by small ubiquitin-related modifier 3 signals its ubiquitination in the nucleus, leading to transactivation of the CLOCK/BMAL1 heterodimer. CLOCK/BMAL1 transactivation, is activated by phosphorylation by casein kinase 1ε and inhibited by phosphorylation by MAPK. Phosphorylation by CK2α regulates BMAL1 intracellular localization and phosphorylation by GSK3B controls BMAL1 stability and primes it for ubiquitination.