Enabling and disabling polo-like kinase 1 inhibition through chemical genetics.

Polo-like kinase 1 (Plk1) is a core regulator of cell division and an emerging target for cancer therapy. Pharmacologic inhibitors of Plk1 exist but affect other kinases, complicating their in vivo validation. To address this, we examined effects of two structurally unrelated Plk1 inhibitors (BI-2536 and TAL) against isogenic human cell lines that solely express wildtype (wt) or analogue-sensitive (as) Plk1 alleles. Unexpectedly, Plk1(as) cells displayed profound biochemical and functional resistance to both inhibitors. Cells that co-express Plk1(wt) and Plk1(as) exhibit loss-of-function phenotypes only when both kinase alleles are inhibited. Resistance to BI-2536 is linked to an intragenic suppressor mutation (C67V) that restores an otherwise invariant valine to the kinase active site. Structural modeling demonstrates that this mutation not only enables Plk1(as) to function in vivo but also occludes BI-2536 from the ATP-binding pocket. Our results reveal the molecular basis of Plk inhibitor selectivity and a potential mechanism for tumor cell resistance.