Competing E3 Ubiquitin Ligases Govern Circadian Periodicity by Degradation of CRY in Nucleus and Cytoplasm

Period determination in the mammalian circadian clock involves the turnover rate of the repressors CRY and PER. We show that CRY ubiquitination engages two competing E3 ligase complexes that either lengthen or shorten circadian period in mice. Cloning of a short-period circadian mutant, Past-time, revealed a glycine to glutamate missense mutation in Fbxl21, an F-box protein gene that is a paralog of Fbxl3 that targets the CRY proteins for degradation. While loss of function of FBXL3 leads to period lengthening, mutation of Fbxl21 causes period shortening. FBXL21 forms an SCF E3 ligase complex that slowly degrades CRY in the cytoplasm but antagonizes the stronger E3 ligase activity of FBXL3 in the nucleus. FBXL21 plays a dual role: protecting CRY from FBXL3 degradation in the nucleus and promoting CRY degradation within the cytoplasm. Thus, the balance and cellular compartmentalization of competing E3 ligases for CRY determine circadian period of the clock in mammals. [Display omitted] ► A short-period circadian mouse mutant is caused by a missense mutation in FBXL21 ► FBXL21 forms an SCF E3 ligase complex for CRY degradation in the cytoplasm ► FBXL21 antagonizes the E3 ligase activity of FBXL3 to protect CRY in the nucleus ► The balance of competing E3 ligases for CRY determines circadian period of the clock FBXL21, an F-box-type ubiquitin E3 ligase, is identified as a regulator of the circadian clock repressor CRYPTOCHROME (CRY) proteins. FBXL21 has the opposite effects on circadian period and CRY stability compared to its homolog, FBXL3, and their combined actions at different subcellular locations ensure stable oscillation of the circadian clock.