mPER1-mediated nuclear export of mCRY1/2 is an important element in establishing circadian rhythm.

Receptor-mediated nucleocytoplasmic transport of clock proteins is an important, conserved element of the core mechanism for circadian rhythmicity. A systematic analysis of the nuclear export characteristics for the different murine period (mPER) and cryptochrome (mCRY) proteins using Xenopus oocytes as an experimental system demonstrates that all three mPER proteins, but neither mCRY1 nor mCRY2, are exported if injected individually. However, nuclear injection of heterodimeric complexes that contain combinations of mPER and mCRY proteins shows that mPER1 serves as an export adaptor for mCRY1 and mCRY2. Functional analysis of dominant-negative mPER1 variants designed either to sequester mPER3 to the cytoplasm or to inhibit nuclear export of mCRY1/2 in synchronized, stably transfected fibroblasts suggests that mPER1-mediated export of mCRY1/2 defines an important new element of the core clock machinery in vertebrates.