Abstract
We identify in this study a 27-amino-acid motif which is conserved between the
Drosophila melanogaster period
protein (PER) and the three mammalian PERs. Characterization of PER lacking this motif (PERΔ) shows that it is important for phosphorylation of
Drosophila
PER by casein kinase Iɛ (CKIɛ;
doubletime
protein or DBT) and CKII. S2 cell assays indicate that the domain also contributes significantly to PER nuclear localization as well as to PER transcriptional repressor activity. These two phenomena appear linked, since PERΔ transcriptional repressor activity in S2 cells was restored when nuclear localization was facilitated. Two less direct assays of PERΔ activity in flies can be interpreted similarly. The separate assay of nuclear import and export suggests that the domain functions in part to facilitate PER phosphorylation within the cytoplasm, which in turn promotes nuclear entry. As there is evidence that the kinases also function within the nucleus to promote transcriptional repression, we suggest that there is a subsequent collaboration between phosphorylated PER and the kinases to repress CLK-CYC activity, probably through the phosphorylation of CLK. This is then followed by additional PER phosphorylation, which occurs within the nucleus and leads to PER degradation.