PRL1 and CK2 collaborate to time morning and evening phase in short photoperiods

The TIMELESS (TIM) phosphatase, Phosphatase of Regenerating Liver-1 (PRL-1), is required for setting behavioral phase especially under short photoperiod conditions. Yet the circadian clock neurons and underlying mechanisms important for these effects are unknown. Here we demonstrate that PRL-1 function in PDF neurons is necessary and sufficient for timing morning and evening phase under short photoperiod (6:18) conditions. In standard (12:12) photoperiod, we find that PRL-1 rescue or blocking PRL-1 knockdown in PDF neurons restores wild-type morning phase. Expression of the TIM kinase shaggy (sgg) in PDF neurons suppresses in an additive manner PRL-1 period lengthening and delayed morning phase further underscoring the role of PDF neurons. In contrast to knockdown in all clock neurons, knockdown of PRL-1 selectively in PDF clock neurons does not alter the timing of morning or evening phase under standard photoperiod conditions, suggesting PRL-1 function in non-PDF neurons may also be important. In contrast to SGG effects, we find that phase delay of evening onset due to expression of a dominant negative version of the TIM kinase CK2 (CK2alphaTik) requires PRL-1. Given that TIM is a substrate of both the PRL-1 phosphatase and CK2 kinase, we hypothesize that PRL-1 dephosphorylation at specific sites may be necessary for subsequent CK2 phosphorylation to time behavior.