Stress activates myosin-II. (A) Schematic for how MIIA conformation and tail phosphorylation are stress-sensitive. (B) Blebbistatin decreases tension in MIIA filaments, and both MEG-01 cells analyzed by flow cytometry (upper) or platelets in fresh bone marrow aspirate (lower) show a similar increase in phosphorylation (n = 25 platelets, ±SEM, *P < .001). (C) MEG-01 cells sheared for 15 minutes show a shear dependent decay in S1943 phosphorylation (n = 4, ±SEM). %, the relative fluorescence intensity rather than stoichiometry. The stress- and time-dependent fit is: %pS1943/MIIA = A+B*exp[(t/τ)*(1-exp(σ/σ₀)], where A = 4.9, B = 9.1, τ = 7.1 min, σ₀ = 1.6 Pa (R² = 0.98, *P < .05). Dashed lines are from this stress-dependent kinetic model. The solid star represents quantification of resting fresh human platelets and the open star represents the value of CD34⁺-derived, CD41⁺ mature MKs (n = 3 donors, ±SEM). (D) High ploidy, mature MKs localize to the perivascular niche, where they extend membrane extensions into the bloodstream within minutes that then fragment within 30 minutes by blood shear.¹  Although these initial fragments may be large and heterogeneous in size, additional exposure to shear can activate MIIA to facilitate cytofission to normal, small platelets. This process depends on MIIA activity and is sensitive to shear stress through loss of pS1943.