Characterization of sqh phosphomutant oligomerization. (A) Phosphorylation of the Drosophila Myo-II RLC sqh on threonine-20 and serine-21 is predicted to activate motor activity and promote bipolar minifilament assembly. (B) Chart summarizing phosphorylation states available to different Sqh phosphomutants. (C) Sqh mutants that mimic phosphorylation result in cytoplasmic Myo-II aggregates. Representative images of XY semisagittal sections and YZ cross sections for live cellularizing embryos. Dashed lines highlight evenness (sqh-TS) or unevenness (sqh-AA) of cellularization front. Arrowheads indicate examples of cytoplasmic Myo-II aggregates. Bar, 20 µm. (D) Basal Sqh aggregates colocalize with Zipper aggregates. Images are fixed embryos stained for Zipper (myosin heavy chain) and endogenous GFP. Bar, 10 µm. (E) Basal Sqh aggregates do not colocalize with F-actin. Images are fixed embryos stained for F-actin (phalloidin) and endogenous GFP. Bar, 5 µm. (F) Phosphomimetic Sqh mutants deplete levels of cytoplasmic Myo-II. Image is a YZ cross section of a live sqh¹; sqh-TS::GFP embryo immediately adjacent to a sqh¹; sqh-EE::GFP embryo. Bar, 20 µm.