Competition between myosin II and β H -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 (β H -spectrin, encoded by the karst gene) in wing imaginal discs. Our results establish that β H -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 β H -spectrin localizes and functions independently of α-spectrin. Instead, β H -spectrin co-localizes with and reciprocally regulates and is regulated by myosin. In vivo and in vitro experiments support a model in which β H -spectrin and myosin directly compete for binding to apical F-actin. This competition can explain the influence of β H -spectrin on cytoskeletal tension and myosin accumulation. It also provides new insight into how β H -spectrin participates in ratcheting mechanisms associated with cell shape change.