Schematic of a native nonmuscle myosin-IIA molecule, its assembly steps, and the purified tail fragments used in this study. (A) A schematic of nonmuscle myosin-IIA shows 2 heavy chains (white and black globular and coiled domains) and the essential and regulatory light chains (ELC and RLC are indicted by arrows). An arrow marks the junction (P836) between the motor head domain and the rod. The arrowhead marks the junction between subfragment 2 (S2) and light meromysoin (LMM) of the rod. The bounded arrow indicates the region (amino acids [aa] 1102-1960) of the wild-type rod used in this study, and an asterisk signifies the relative position of the 4 mutations studied (R1165C, D1424N, E1841K, R1933Stop, reading from left to right). (B) Assembly steps of nonmuscle myosin-IIA. The rod of individual heavy chains fold into an α-helical structure that dimerizes with another heavy chain, forming a coiled coil. A cross-section through a coiled coil, appearing below the diagram, shows the relationship of amino acids in the characteristic heptad repeat. To form higher order assemblies, such as contractile filaments, dimers must laterally associate with one another in proper register. Periodic charged regions along the rod play important roles during this alignment. The α-helix and coiled-coil diagrams are reprinted from Figures 3-6 and 3-9 of Lodish et al¹⁴  with permission. (C) Here, 5 μg each recombinantly purified protein were subjected to electrophoresis on an acrylamide gel and Coomassie stained (lane 1 indicates wild type; lane 2, R1165C; lane 3, D1424N; lane 4, E1841K; and lane 5, R1933Stop). Two to 3 minor bands, probably breakdown products, can be seen in each lane.