Abstract
Scaffolding is a fundamental principle of cell signaling commonly thought to involve multi-domain proteins that tether different components of a pathway together into a complex 1,2. We now report an alternative mechanism for scaffolding that is necessary for RhoA-mediated contractile signaling. We find that anillin binding stabilizes active, GTP-RhoA, and promotes contractility at both the epithelial zonula adherens (ZA) and the cytokinetic furrow. However, anillin does not conform to the classical picture of a multi-domain tether, since its RhoA-binding AH domain alone was sufficient to promote contractile signaling. Moreover, anillin competes with contractile effectors for a common site on RhoA, presenting the conundrum of how an inhibitory interaction can otherwise promote signaling. To explain this, we propose that inactivation of RhoA is non-Poissonian, having a rate that increases with time, unless the process is reset via transient binding to anillin. Repeated cycles of binding and un-binding therefore increase cortical residence times of non-sequestered GTP-RhoA and hence the probability of engaging contractile effectors. We identify the modification of the local lipid environment as a potential mechanism underlying such non-Poisson statistics, and demonstrate agreement with a minimal cellular system. Finally, we show that Myosin II anchors anillin at the cortex to form a feedback pathway that enhances RhoA signaling. This new paradigm of scaffolding is a regulatory analogue of kinetic proofreading and may be employed by other binding proteins that do not fit the classical picture.
