A role for the proteasome in the light response of the timeless clock protein

The cyclic expression of the period (PER) and timeless (TIM) proteins is cr itical for the molecular circadian feedback Loop in Drosophila. The entrain ment by Light of the circadian clock is mediated by a reduction in TIM Leve ls. To elucidate the mechanism of this process, the sensitivity of TIM regu lation by Light was tested in an in vitro assay with inhibitors of candidat e proteolytic pathways. The data suggested that TIM is degraded through a u biquitin-proteasome mechanism. In addition, in cultures from third-instar L arvae, TIM degradation was blocked specifically by inhibitors of proteasome activity. Degradation appeared to be preceded by tyrosine phosphorylation. Finally, TIM was ubiquitinated in response to Light in cultured cells.