Role of cysteine amino acid residues on the RNA binding activity of human thymidylate synthase

The role of cysteine sulfhydryl residues on the RNA binding activity of human thymidylate synthase (TS) was investigated by mutating each cysteine residue on human TS to a corresponding alanine residue. Enzymatic activities of TS:C43A and TS:C210A mutant proteins were nearly identical to wild-type TS, while TS:C180A and TS:C199A mutants expressed >80% of wild-type enzyme activity. In contrast, TS:C195A was completely inactive. Mutant proteins, TS:C195A, TS:C199A and TS:C210A, retained RNA binding activity to nearly the same degree as wild-type human TS. RNA binding activity of TS:C43A was reduced by 30% when compared to wild-type TS, while TS:C180A was completely devoid of RNA binding activity. In vitro translation studies confirmed that mutant proteins TS:C43A, TS:C195A, TS:C199A and TS:C210A, significantly repressed human TS mRNA translation, while TS:C180A was unable to do so. To confirm the in vivo significance of the cysteine sulfhydryl residue, mutant proteins TS:C180A and TS:C195A were each expressed in human colon cancer HCT-C18:TS(-) cells that expressed a functionally inactive TS. A recombinant luciferase reporter gene under the control of a TS-response element was cotransfected into these same cells, and luciferase activity increased in the presence of the TS:C195A mutant TS protein to a level similar to that observed upon expression of wild-type TS protein. In contrast, luciferase activity remained unchanged in cells expressing the TS:C180A mutant protein. Taken together, these findings identify Cys-180 as a critical residue for the in vitro and in vivo translational regulatory effects of human TS.