Fig. 4. Synthetic GR₂₀ and PR₂₀ peptides bind nucleoli and kill cells.

(A) Peptides containing 20 repeats of GR or PR (GR₂₀ or PR₂₀, respectively) were synthesized to contain an HA epitope tag and applied to cultured U2OS cancer cells (left panels) or human astrocytes (right panels). Cells were fixed and stained with either the HA reacting antibody (green signal) or an antibody to the nucleolar protein Fibrillarin (red signal). Both GR₂₀ and PR₂₀ synthetic peptides associated prominently with nucleoli. Measurements of U2OS cell viability revealed toxicity in response to both PR₂₀ (B) and GR₂₀ (C) synthetic peptides. Cell viability was measured at 72 or 12 hours after initial treatment of PR₂₀ or GR₂₀, respectively. In the case of GR₂₀ peptide, the medium was replaced every 2 hours to supplement fresh peptide. The PR₂₀ and GR₂₀ synthetic peptides killed U2OS cells with IC₅₀ levels of 5.9 and 8.4 μM, respectively.