Peptide C-617F accelerates megakaryocyte PPF. (A) Sequences of C-617V and C-617F peptides with N-terminal CPPs. (B) Immunofluorescence images of α-tubulin in HEK293T cells treated with TAMRA-labeled C-617F or TAMRA-labeled C-617V. Scale bars, 10 μm. (C) Immunoblot analysis of Stat3 and JAK2 phosphorylation levels in platelets incubated with or without 10 μM peptide C-617F/C-617V for 20 minutes at 37°C. (D) Immunoblot analysis of Stat3 phosphorylation levels in WT or AGK-null platelets incubated with or without 10 μM peptide C-617F for 20 min at 37°C. (E) Immunofluorescence images of fetal liver–derived megakaryocytes stimulated with PBS, 5 μM peptide C-617F, or 10 μM peptide C-617F and stained with α-tubulin antibodies (Alexa Fluor 488) and DAPI. Scale bars, 50 μm. Statistics for the PPF-megakaryocytes/total megakaryocytes among megakaryocytes stimulated with PBS, 5 μM peptide C-617F, or 10 μM peptide C-617F are shown (n = 10, **P < .01). (F) Platelets of Agk^f/f and Agk^−/− mice were eliminated by tail IV injection of anti-CD42b (2 µg/g). BM injection of the peptide C-617F (20 µg/g) was carried out 36 hours later, and platelet counts were monitored with a HEMAVET automated hematologic analyzer at different time points (n = 3; *P < .05, **P < .01). (G) Schematic of peptide 617F-mediated augmentation of JAK2/Stat3 signaling resulting from increased interaction between AGK and JAK2 in megakaryocytes.