A new model for coordinating the functions of TIMELESS at the replication fork

TIMELESS is an essential protein that supports a multitude of various cell functions, from replication fork progression through intrinsic barriers in the genome and DNA damage checkpoint to double strand break repair, transcription, and circadian rhythm. How TIMELESS coordinates its various roles at the replication fork and in DNA damage response, and how its canonical position at the leading edge of the replication fork could facilitate its role in DNA damage checkpoint, remain poorly understood. Using an auxin-inducible degron system, we show that TIMELESS-depleted cells exhibited S phase entry defects, and compromised chromatin loading of CLASPIN and TIPIN - the other two components of the Fork Protection Complex (FPC). We further show that FPC chromatin loading was concurrent with the activation of the replicative helicase, but also required proficient DNA synthesis. Proximity labelling experiments suggested the existence of more than one molecule of TIMELESS per replication fork. TIMELESS interaction with the replicative helicase was essential for the speed of replication fork progression, but not for the role of TIMELESS in the activation of the replication checkpoint. Our data propose a new model for coordinating essential functions of TIMELESS in replication fork progression and checkpoint activation.