The role of phosphorylation and degradation of hPER protein oscillation in normal human fibroblasts.

The circadian expression in Drosophila of clock gene products, such as PER and TIM, is thought to be important for driving overt rhythms. The constitutive expression of per by the heat-shock or rhodopsin promoters restores rhythmicity of the null allele of per, suggesting that per mRNA cycling may not be required for protein cycling or for locomotor rhythms. Furthermore, the constitutive expression of tim mRNA also supports protein cycling and behavioural rhythms in tim mutant flies. Other reports have also shown that eliminating the oscillations of PER and TIM proteins by their over-expression abrogated circadian rhythmicity. These data indicate that the circadian rhythmic expression of PER and TIM proteins is also important like their rhythmic mRNA expression in Drosophila. To compare the molecular mechanism of circadian clocks in divergent species, we report here cloning circadian mRNA and protein expression profiling of human clock genes in normal human fibroblasts. Circadian oscillations of hPer1, hPer2, hPer3, hBMAL1 and hCry2 mRNA expression were observed in serum-stimulated normal human fibroblasts. The serum shock of human fibroblasts also caused daily oscillations in the amount and size of human PER proteins as was shown using our novel antibodies. Inhibitor studies indicate that phosphorylation and degradation of PER proteins is an important process in the human molecular clock.