Abstract
The timing of mammalian circadian rhythm is determined by interlocking negative and positive transcriptional feedback loops that govern the cyclic expression of both clock regulators and output genes. In mammals, nuclear localization of the circadian regulators PER1-3 is controlled by multiple mechanisms, including multimerization with PER and CRY proteins. In addition, nuclear entry of mammalian PER1 (mPER1) can be regulated by a phosphorylation-dependent masking of its nuclear localization signal. Here we present evidence suggesting that nuclear localization of PER proteins is a dynamic process determined by both nuclear import and previously unrecognized nuclear export pathways. Examination of the subcellular localization of a series of truncated mPER1 proteins demonstrated that cytoplasmic localization is mediated by an 11-amino acid region with homology to leucine-rich nuclear export signals (NESs). Similar sequences were identified in mPER2 and mPER3 as well as in several insect PER proteins. The putative NESs from mPER1 and mPER2 were able to direct cytoplasmic accumulation when fused to a heterologous protein. Mutations in conserved NES residues and the nuclear export inhibitor leptomycin B each blocked the function of the NES. Full-length mPER1 was also exported from microinjected Xenopus laevis oocyte nuclei in an NES-dependent manner. The presence of a functional NES in mPER1 and mPER2 as well as related sequences in a variety of other PER proteins suggests that nuclear export may be a conserved and important feature of circadian regulators.