The relentless ticking of the clock leaves some of us struggling with our time keeping more than others, but, at the cellular level, our circadian clocks are finely controlled. Regulatory feedback loops that involve clock proteins preserve cellular circadian rhythmicity, and Paolo Sassone-Corsi and colleagues have now shown that regulation of a key clock protein, BMAL1 (brain and muscle aryl hydrocarbon receptor nuclear transporter (ARNT)-like protein-1), is likely to occur through modification with SUMO (small ubiquitin-like modifier).

BMAL1 and its binding partner CLOCK regulate the expression of genes that are involved in the circadian-clock mechanism, such as PER1 (period homologue-1), through binding a regulatory DNA sequence known as the E-box. The BMAL1 protein has several lysine residues that are conserved among members of the same protein family as well as across species. And computer modelling analysis shows that these residues closely match the consensus motif for sumoylation. Paolo Sassone-Corsi and colleagues found that BMAL1 is successfully sumoylated in COS1 cells, and showed by site-directed mutagenesis that K259 is the main in vivo sumoylation site.

Liver tissues were treated with an inhibitor of SUMO proteases to stabilize sumoylated proteins, and a sumoylated form of BMAL1 was detected, the abundance of which oscillates in a circadian manner. Phosphorylation of BMAL1 increases in parallel with its sumoylation and this also coincides with the induction of circadian-clock genes such as PER1.

Although CLOCK was known to mediate BMAL1 phosphorylation, the authors found that it also mediates BMAL1 sumoylation. But what is the function of BMAL1 sumoylation? In contrast to wild-type BMAL1, the levels of BMAL1(K259R) do not oscillate in a circadian manner. Moreover, BMAL1(K259R) is more than twice as abundant as its wild-type counterpart. So protein degradation is likely to have an important function in the control of BMAL1.

Bmal1−/− mouse embryonic fibroblasts have altered circadian oscillation of an E-box-controlled gene ( Dbp ). Virally expressed BMAL1 rescues the circadian oscillation of this gene, but expressing BMAL1(K259R) generates an altered, shorter period of Dbp rhythmicity.

So it seems that sumoylation has an important role in the circadian expression and function of BMAL1, which reveals a previously unknown facet of circadian-clock regulation. The authors suggest that future studies should look at identifying elements of the SUMO pathway that might be selective for circadian-clock components.