Role of the RGS (regulator of G protein signaling) domain of Axin in vertebrate axis formation

Abstract

The Wnt signaling network has major roles in developmental processes, human diseases and stem cell biology. A protein complex that includes Axin, APC and GSK3 tightly regulates this signaling. Axin, a key player in this complex, contains four critical domains: a β‐catenin binding domain, a GSK binding domain, an RGS domain and a DIX domain. Our study focuses on the role of the Axin‐RGS domain. Typically, RGS domains contain a conserved asparagine (Asn), critical for the ability to bind heterotrimeric G‐proteins. Interestingly, we find that the Axin‐RGS domain encodes a glutamine (Gln) at the equivalent position, suggesting a novel function for Axin‐RGS. We tested whether the Axin‐RGS domain functions in the context of Wnt signaling. We generated point mutations that converts the original Gln to Asn, predicted to increase affinity for Gα, and that converts Gln to Ala, predicted to abolish binding activity. We tested for the ability of the mutant forms to rescue Axin function in masterblind (mbl) zebrafish mutant embryos and in antisense morpholino knockdown of Axin in zebrafish and frog (Xenopus laevis). We found that the mutant constructs showed a differential ability to restore normal development in fish and frog embryos. These results suggest that the Axin‐RGS domain may be playing a role in regulating zygotic Wnt signaling.Grant Funding SourceRoy J. Carver Charitable Trust