REGULATION OF NUCLEAR ENTRY OF THE DROSOPHILA CLOCK PROTEINS PERIOD AND TIMELESS

Two genes, period (per) and timeless (tim), are essential for circadia n rhythmicity in drosophila. The encoded proteins (PER and TIM) physic ally interact. Here, it is shown that TIM and PER accumulate in the cy toplasm when independently expressed in cultured (S2) Drosophila cells . However, the proteins move to the nuclei of these cells if coexpress ed. Domains of PER and TIM have been identified that block nuclear loc alization of the monomeric proteins. In vitro protein interaction stud ies indicate that the sequence inhibiting the nuclear accumulation of PER forms a binding site for TIM. The results indicate a mechanism for controlled nuclear localization in which suppression of cytoplasmic l ocalization is accomplished by direct interaction of PER and TIM. No o ther clock functions are required for nuclear localization. The findin gs suggest that a checkpoint in the circadian cycle is established by requiring cytoplasmic assembly of a PEW TIM complex as a condition for nuclear transport of either protein.