A structural model for phosphorylation control of Dictyostelium myosin II thick filament assembly

Myosin II thick filament assembly in Dictyostelium is regulated by phosphor ylation at three threonines in the tail region of the molecule. Converting these three threonines to aspartates (3xAsp myosin II). which mimics the ph osphorylated state, inhibits filament assembly in vitro, and 3xAsp myosin I I fails to rescue myosin II-null phenotypes. Here we report a suppressor sc reen of Dictyostelium myosin II-null cells containing 3xAsp myosin II, whic h reveals a 21-kD region in the tail that is critical for the phosphorylati on control. These data, combined with new structural evidence from electron microscopy and sequence analyses, provide evidence that thick filament ass embly control involves the folding of myosin II into a bent monomer, which is unable to incorporate into thick filaments. The data are consistent with a structural model for the bent monomer in which two specific regions of t he tail interact to form an antiparallel tetrameric coiled-coil structure.