Mammalian homologues of the plant Tousled gene code for cell-cycle-regulated kinases with maximal activities linked to ongoing DNA replication

The Tousled (TSL) gene of the plant Arabidopsis thaliana encodes a serine/t hreonine kinase that is essential for proper flower development. Here we re port the cloning and characterization of two human putative homologues of t he Arabidopsis TSL gene, termed TLK1 and TLK2 (Tousled-like kinase), At the protein level, the two human Tlks share 84% sequence similarity with each other and almost 50% with Arabidopsis Tsl, Furthermore, nuclear localizatio n signals and predicted coiled-coil regions are conserved in the N-terminal domains of all three kinases, The mammalian Tlks share several functional properties with plant Tsl, including a broad expression, a propensity to di merize and autophosphorylate, and a preference for similar substrates. Most interestingly, human Tlks are cell-cycle-regulated enzymes, displaying max imal activities during S phase. Whereas protein levels are virtually consta nt throughout the cell cycle, both Tlks appear to be regulated by cell-cycl e-dependent phosphorylation, Drug-induced inhibition of DNA replication cau ses a rapid loss of Tlk activity, indicating that Tlk function is tightly l inked to ongoing DNA replication. These findings provide the first biochemi cal clues as to the possible molecular functions of Tlks, a highly conserve d family of kinases implicated in the development of multicellular organism s.