The Tousled kinase (Tsl) was initially identified in Arabidopsis thaliana via a mutation responsible for aberrant floral organ.. There are two mammalian orthologues of Tsl and one orthologue in C. elegans, TLK-1; the human kinases are nuclear proteins with maximal activity tied to ongoing DNA replication during S-phase. Very few substrates of TLK have been described with the most reported being the chromatin assembly factor ASF. Aside from its S-phase functions, we have shown that TLK-1 also contributes to chromosome segregation as a substrate and activator of the AIR-2/Aurora B kinase. AIR-2-dependent phosphorylation of TLK-1 is detectable at the centrosomes, kinetochore (KT), and KT-microtubules (MTs) from early prophase to metaphase. TLK-1 depletion phenotypes include embryonic lethality, a spindle assembly checkpoint (SAC)-dependent mitotic delay, chromosome segregation defects, and distended nuclei in late multicellular embryos. To further determine the consequences of TLK-1 depletion in developing embryos, we used live-cell analysis of early embryos subjected to
tlk-1(RNAi). Our analysis revealed a spindle rotation defect in 50% of a TLK-1-depleted population during the first mitotic division. More specifically, the centrosome-pronuclear complex rotation in TLK-1-depleted embryos is significantly delayed with respect to nuclear envelope breakdown. This striking phenotype suggests a nascent role for a known chromatin regulator, TLK-1, in regulating microtubule behavior during mitosis, and the epistatic relationships between
tlk-1 and other genes required for spindle rotation are being investigated. It has been reported that P0 embryos depleted of regulatory dynein subunits exhibit an inability to correctly rotate the centrosome-pronuclear complex in a manner strikingly similar to that of the TLK-1-depleted- P0 embryos, suggesting that TLK-1 functionality may influence dynein-dependent processes in the early embryo. Interestingly, co-depletion of TLK-1 with certain dynein subunits results in an inability to completely separate centrosomes and correctly rotate the mitotic spindle, an effect that forces MTs emanating from centrosomes to attach KTs in a monopolar-like fashion. With these data taken together, we hypothesize that TLK-1 is affecting vital cytoskeletal processes relating to centrosome dynamics and other dynein-related processes during mitosis, and relating the established roles of TLK-1 to its functions in cytoskeletal/spindle dynamics and cell cycle regulation will shed light on the consequences of TLK-1 action during development.