Time-lapse microscopy of a spreading MIIB (myosin II B)-deficient cell cell expressing RLC-D,D–GFP (a phospho-mimetic mutant of the RLC) on fibronectin. Arrows point to clusters, or proto-bundles, denoted by the accumulation of MIIA (the only isoform present in the knockdowns, labeled with the RLC-D,D–GFP) that form and disassemble rapidly. Images were captured in TIRF mode using an inverted microscope (IX70; Olympus) coupled to a CCD camera (Retiga Exi; Qimaging). Live samples were acquired at 37°C using CCM1 medium. This movie corresponds to Fig. 1 D. and Video 2 from J Cell Biol 2011 193:381-396.