Competition between myosin II and βH-Spectrin regulates cytoskeletal tension

Abstract

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