Rac1 modulation of the apical domain is negatively regulated by βHeavy-spectrin

Epithelial polarity and morphogenesis require the careful coordination of signaling and cytoskeletal elements. In this paper, we describe multiple genetic interactions between the apical cytoskeletal protein βH and Rac1 signaling in Drosophila: activation of Rac1 signaling by expression of the exchange factor Trio, is strongly enhanced by reducing βH levels, and such reductions in βH levels alone are shown to cause an increase in GTP-Rac1 levels. In contrast, co-expression of a C-terminal fragment of βH (βH33) suppresses the Trio expression phenotype. In addition, sustained expression of βH33 alone in the eye induces a strong dominant phenotype that is similar to the expression of dominant negative Rac1N17, and this phenotype is also suppressed by the co-expression of Trio or by knockdown of RacGAP50C. We further demonstrate that a loss-of-function allele in pak, a Rac1 effector and negative regulator of βH′ dominantly suppresses larval lethality arising loss-of-function karst (βH) alleles. Furthermore, expression of constitutively active Pakmyr in the larval salivary gland induces expansion of the apical membrane and destabilization of the apical polarity determinants Crumbs and aPKC. These effects resemble a Rac1 activation phenotype and are suppressed by βH33. Together, our data suggest that apical proteins including βH are negatively regulated by Rac1 activation, but that Rac1 signaling is also suppressed by βH through its C-terminal domain. Such a system would be bistable with either Rac1 or βH predominant. We suggest a model for apical domain maintenance wherein Rac1 down-regulation of βH (via Pak) is opposed by βH-mediated down-regulation of Rac1 signaling.