TIMe to be degraded

Abstract

Recently, much attention has been drawn to research of the circadian clock. One of the model organisms studied is the fruitfly Drosophila melanogaster, where the cyclic expression of the period (PER) and timeless (TIM) proteins is crucial for the molecular circadian feedback loop. When protein levels are high, negative feedback occurs, resulting in low levels of RNA. Degradation of proteins eliminates the feedback and allows RNA levels to rise once again. Now, Naidoo et al.1xA role for the proteasome in the light response of the timeless clock protein. Naidoo, N. et al. Science. 1999; 285: 1737–1741Crossref | PubMed | Scopus (190)See all References1 demonstrate that TIM is degraded through the ubiquitin–proteasome pathway.Since TIM protein levels are reduced by light, the authors developed an in vitro degradation assay incubating TIM protein immunoprecipitated from fly heads with head extracts from light-pulsed or control flies. Only the head extract from light-pulsed animals was able to induce TIM degradation, even when it was prepared from tim0 flies, which lack endogenous TIM. Several proteasome inhibitors (ALLN, MG115 and MG132) and ubiquitin depletion of the extracts prevented TIM degradation. To confirm this finding, the authors examined TIM expression directly in the lateral neurons (clock cells) in an ex vivo system. Both MG115 and lactacystin (another proteasome inhibitor) suppressed the light-induced TIM degradation. Moreover, when TIM and ubiquitin were coexpressed in S2 cells, TIM was ubiquitinated. It was also shown that light-induced TIM tyrosine phosphorylation is a prerequisite for degradation. TIM also contains seven PEST sequences, which usually serve as phosphorylation/degradation signals. The nature of the participating kinase(s) is unknown.This paper shows us that the proteasome field is broadening again, encompassing now circadian clocks too. It is intriguing that the proteolytic activities of the proteasome and/or the activity of the ubiquitination cascade are somehow altered by circadian rhythms.