Myosin II is essential for efficient pore expansion and essential for constriction. Time-lapse videos of representative LSVs (SRRF intensity-projection) from WT untreated SG undergoing membrane crumpling (MC; top), and from ZipperKD (myosin II heavy chain) and Y-27632 treated SG, undergoing stalling (middle and bottom; corresponds to data presented in Fig. 3, B and C). The video runs twice, the first run is unmarked and in the second run, the pores are outlined by two white curved lines on the Glue-GFP image (middle column), and the maximal diameter and its occurring time is noted on the top for each of the three LSVs. In the WT, untreated SG, the pore expands (up to 1.8 µm at 00:48) and constricts quickly, as the LSV undergoes membrane crumpling until it cannot be resolved (03:28). In the stalled LSV from ZipperKD SG, the pore expands slower (reaching a maximal diameter of 1.2 µm at 01:15) then it constricts partly and slowly, still visible at the end of the video almost 5 min after fusion (04:45). Lastly, the pore of the stalled LSV from the Y-27632 treated SG, expands with the slowest rate out of the three LSVs presented (reaching a maximal pore diameter of 1.5 µm at 03:12) and then arrests without constricting, still seen expanded almost 6 min after fusion (05:52). Time mm:ss; relative to fusion, seen in the second frames. Video frames were taken every 16 (for WT and Y-27632) or 15 s (for ZipperKD). LifeAct-Ruby (magenta; left column; expression driven by c135-GAL4) and Glue-GFP (green; middle column; endogenous promoter). The merged image of both imaging channels is presented in the right column. Scale bars = 1 µm.
