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Abstract
Tissue-specific overexpression of the glycogen synthase kinase-3 (GSK-3) ortholog
shaggy (sgg) shortens the period of the Drosophila circadian locomotor activity cycle.
The short period phenotype was attributed to premature nuclear translocation of the
PERIOD/TIMELESS heterodimer. Reducing SGG/GSK-3 activity lengthens period, demonstrating
an intrinsic role for the kinase in circadian rhythmicity. Lowered sgg activity decreased
TIMELESS phosphorylation, and it was found that GSK-3 beta specifically phosphorylates
TIMELESS in vitro. Overexpression of sgg in vivo converts hypophosphorylated TIMELESS
to a hyperphosphorylated protein whose electrophoretic mobility, and light and phosphatase
sensitivity, are indistinguishable from the rhythmically produced hyperphosphorylated
TIMELESS of wild-type flies. Our results indicate a role for SGG/GSK-3 in TIMELESS
phosphorylation and in the regulated nuclear translocation of the PERIOD/TIMELESS
heterodimer.