Analysis of Moesin function in Rho1 regulation, cell survival, and epithelial integrity

ERM proteins have been characterized as cytoskeletal linkers that connect the underlying actin cytoskeleton to the apical cell membrane, and in addition they been shown to be involved in cell signaling events. Specifically, they have been characterized as regulators of the activity of the GTPase RhoA. Drosophila contains a single ERM protein, Moesin, which is involved in diverse cellular processes, some of which involve negative regulation of RhoA activity and others that involve regulation of the actin cytoskeleton. We show here that Moesin controls cell survival by negatively regulating the amount of RhoA protein at the cell cortex. Independent of its GTP-binding state, high levels of RhoA at the cortex trigger a cell death pathway which involves activation of JNK signaling. To determine how Moesin negatively regulates RhoA levels at the cortex the function of a RhoGAP protein, Conundrum, was investigated. Moesin interacts with Conundrum, and the subcellular localization of Conundrum at the cortex is dependent on Moesin. conundrum mutant alleles are homozygous viable, suggesting that Moesin may modulate RhoA levels at the membrane by regulating multiple RhoA regulators.In Drosophila, at least three Moesin isoforms exist. We have investigated the function of one of the isoforms, Moesin-RB, which is less abundant than the predominant form. This isoform potentially regulates the activity of the predominant isoform as co-expression of Moesin-RB and the predominant Moesin isoform results in the predominant form assuming the localization of Moesin-RB. Additionally, expression of an activated form of Moesin-RB dramatically decreases endogenous activated Moesin, suggesting that Moesin-RB and the predominant form may compete for binding sites, thus limiting the amount of the predominant form that can be activated. Together our results show that Moesin is an essential regulator of RhoA activity at the cell cortex and that Moesin can itself be regulated by alternative splicing.