Timeless (gene)

Timeless (tim) is a gene in Drosophilia which encodes a protein, TIM, that regulates circadian rhythm. The human timeless protein (hTIM) has been shown to be required for the production of electrical oscillations output by the suprachiasmatic nucleus (SCN), the major clock governing all tissue-specific circadian rhythms of the body. hTIM also interacts with the products of major clock genes CLOCK, BMAL, PER1, PER2 and PER3. A decrease in the expression of hTIM brings about a drastic decrease in the levels of the expression of the aforementioned canonical genes.

The hTIM protein also exhibits a circadian regulation of expression. Interestingly, its closest phylogenetic relatives are cell-cycle related proteins and the hTIM protein itself has been showed to play an integral role in two cell cycle checkpoints: G2/M and intra-S checkpoints. A null mutation in the Timeless protein ortholog resulted in embryonic lethality in C. elegans and mice. The hTIM also has a cell cycle dependent oscillation that is low in G0, G1 phases and high in G2, S1 and M, with the highest expression occurring in S1 phase.

hTIM mediates the reaction between two proteins which subsequently results in an arrested phase in the cell cycle that allows DNA repair to occur. The G2/M checkpoint prevents cells from entering mitosis when DNA is damaged; thereby producing an opportunity for DNA repair and stopping the proliferation of damaged cells. HU (hydroxyurea) and UV light are both DNA replication inhibitors and when cells are exposed to either chemical, they produce breaks in the double stranded DNA. Within the nucleus there is a protein kinase called ATR (Ataxia-Telangiectasia mutated and Rad 3 related) that’s recognizes damaged DNA. hTIM interacts with a subunit on the ATR known as ATRIP to bring about the phosphorylation of Checkpoint kinase protein 1 (Chk1). Chk1 is required for normal cell proliferation and survival. When this protein is phosphorylated, cell cycle is arrested at the G2/M phase which allows for DNA repair, or if the damage is too extensive, apoptosis. Hence, hTIM acts as an important mediator between ATR and Chk1 and helps to prevent the proliferation of cells with damaged DNA.

hTIM protein plays an essential yet undetermined role in the intra-S checkpoint system In the intra-S checkpoint, stalled replication forks after dNTP pool depletion or DNA damage activates a signal transduction pathway that inhibits the firing of new origins of replications on DNA strand elsewhere. This checkpoint is essential in protecting stalled replication forks from pathological rearrangements that can result from unscheduled recombination. When hTIM is down regulated, the intra-S checkpoint is seriously compromised with continuous firing of replication origins in the presence of replication blocks, which results in unabated DNA synthesis. The timeless protein is thought to directly connect the cell cycle with the circadian rhythm in mammals. In this model called a “direct coupling” the two cycles share a key protein whose expression exhibits a circadian pattern. It should be noted that the circadian cycle operates normally in the absence of the cell cycle, such as the circadian cycle of non-dividing neural, muscle and liver cells.