Characterization of Myosin II in N. crassa. (A) Schematic representation of N. crassa myo2 gene structure and the corresponding encoded protein domains (drawn to scale). The three coding exons (I, II, and III), two small introns, and 3′ UTR are indicated. SH3, SH3 domain; MH, myosin head (motor domain); MT, myosin tail. Numbering indicates the corresponding amino acid residues within the protein sequence. (B) Septa in a wild-type N. crassa strain. The wild-type FGSC#9719 strain was labeled with FM4-64 to visualize membranes and septa. White arrows indicate positions of septa. The image was created from 11 overlapping individual confocal images by merging and cropping in Adobe Photoshop. Bar, 50 µm. (C) Septation is defective in a heterokaryon knockout strain of the myosin II homologue, NCU00551. NCU00551 deleted heterokaryons were labeled with FM4-64 to visualize membranes and septa. The image was created from 12 overlapping individual confocal images by merging and cropping in Adobe Photoshop. Bar, 50 µm. (D) Myo2-GFP localized to a discreet region on the hyphal cortex, assembled into a contractile ring, and persisted at the ring throughout septation. The time is indicated in minutes; arrowhead indicates the region of preassembly accumulation. Bar, 5 µm. See also Video 1. (E) A kymograph showing the decrease in ring diameter over time within a representative hypha expressing Myo2-GFP (green) and membranes labeled with FM4-64 (red) and a merge of the two.