Competition between myosin II and βH-Spectrin regulates cytoskeletal tension

Spectrins are membrane cytoskeletal proteins generally thought to function as heterotetramers comprising two α-spectrins and two β-spectrins. They influence cell shape and Hippo signaling, but the mechanism by which they influence Hippo signaling has remained unclear. We have investigated the role and regulation of the Drosophila β-heavy Spectrin (βH-Spectrin, encoded by the karst gene) in wing imaginal discs. Our results establish that βH-Spectrin regulates Hippo signaling through the Jub biomechanical pathway due to its influence on cytoskeletal tension. While we find that α-Spectrin also regulates Hippo signaling through Jub, unexpectedly, we find that βH-Spectrin localizes and functions independently of α-Spectrin. Instead, βH-Spectrin co-localizes with and reciprocally regulates and is regulated by myosin. In vivo and in vitro experiments support a model in which βH-Spectrin and myosin directly compete for binding to apical F-actin. This competition can explain the influence of βH-Spectrin on cytoskeletal tension and myosin accumulation. It also provides new insight into how βH-Spectrin participates in ratcheting mechanisms associated with cell shape change.