A conserved chronobiological complex times C. elegans development.

The mammalian protein PERIOD (PER) and its C. elegans orthologue LIN-42 have been proposed to constitute an evolutionary link between two distinct, circadian and developmental, timing systems. While the function of PER in animal circadian rhythms is well understood molecularly and mechanistically, this is not true for LIN-42's function in timing rhythmic development, reflected in C. elegans molting cycles. We observed arrhythmic molts upon combined deletion of a region comprising two distinct sequence elements previously termed SYQ and LT. This region functions as a casein kinase I (CK1)-binding domain (CK1BD) mediating stable binding to KIN-20, the C. elegans CK1δ/ε orthologue. CK1 phosphorylates LIN-42, and the CK1BD sub-domains SYQ/CKBD-A and LT/CKBD-B play distinct roles in controlling CK1-binding and kinase activity in vitro. KIN-20 and the LIN-42 CK1BD are required for proper molt timing in vivo, and loss of LIN-42 binding or of the phosphorylated LIN-42 tail impairs nuclear accumulation of KIN-20. These findings indicate that LIN-42/PER and KIN-20/CK1 form a functionally conserved signaling module of two distinct chronobiological systems.