FAD Regulates CRYPTOCHROME Protein Stability and Circadian Clock in Mice

Abstract

Summary: The circadian clock generates biological rhythms of metabolic and physiological processes, including the sleep-wake cycle. We previously identified a missense mutation in the flavin adenine dinucleotide (FAD) binding pocket of CRYPTOCHROME2 (CRY2), a clock protein that causes human advanced sleep phase. This prompted us to examine the role of FAD as a mediator of the clock and metabolism. FAD stabilized CRY proteins, leading to increased protein levels. In contrast, knockdown of Riboflavin kinase (Rfk), an FAD biosynthetic enzyme, enhanced CRY degradation. RFK protein levels and FAD concentrations oscillate in the nucleus, suggesting that they are subject to circadian control. Knockdown of Rfk combined with a riboflavin-deficient diet altered the CRY levels in mouse liver and the expression profiles of clock and clock-controlled genes (especially those related to metabolism including glucose homeostasis). We conclude that light-independent mechanisms of FAD regulate CRY and contribute to proper circadian oscillation of metabolic genes in mammals. : Hirano et al. examine a mechanism in which a co-factor, FAD, regulates CRYPTOCROME proteins and circadian regulation of metabolism. FAD stabilizes CRY proteins by competing with the CRY ubiquitin E3 ligase. Knockdown of Rfk, a biosynthetic enzyme of FAD, alters the expression profiles of CRY and clock-controlled metabolic genes. Keywords: circadian clock, circadian rhythms, CRYPTOCHROME, CRY, FAD, metabolism, FBXL3, Riboflavin kinase, protein degradation