Abstract
Circadian rhythm serves an essential role in numerous physiological functions. Circadian oscillations are organized by circadian clock components at the molecular level. The precision of the circadian clock is controlled by transcriptional-translational negative feedback loops, as well as post-translational modifications of clock proteins, including ubiquitination; however, the influence of E3 ligases on clock protein ubiquitination requires further investigation. The results of co-immunoprecipitation and immunofluorescent localization, indicated that the endoplasmic reticulum transmembrane E3 ubiquitin ligase HRD1, encoded by the synoviolin 1 gene, interacted with brain and muscle ARNT-like 1 (BMAL1) and enhanced BMAL1 protein ubiquitination. In addition, the results of western blotting and reverse transcription-quantitative PCR suggested that HRD1 promoted K48-associated polyubiquitination of BMAL1 and thus mediated its degradation via the ubiquitin-proteasome system. Furthermore, gene knockdown and gene overexpression assays revealed that HRD1-dependent degradation of BMAL1 protein regulated the expression of BMAL1 target genes and the amplitude of circadian oscillations in mammalian cells. The findings of the current study indicate that HRD1 may influence the regulation of circadian rhythm via modulation of BMAL1 stability.