Correct temporal and spatial expression of Wnt target genes is mediated via tight control of the effector beta-catenin. In the default state, beta-catenin is targeted for degradation by a complex consisting of Axin, the kinase GSK3 , and APC. Wnt signaling inhibits this complex via Dishevelled (Dsh), allowing the release of beta-catenin, which in turn interacts with Tcf transcription factors to activate Wnt target gene expression. In C.elegans a similar pathway relayed by EGL-20/Wnt, LIN-17/Frizzled, MIG-5/Dsh, BAR-1/ beta-catenin and POP-1/Tcf controls the expression of target genes such as
mab-5. The mechanisms that control BAR-1 intracellular levels are unknown. Surprisingly, SGG-1/GSK3 and the APC-related protein APR-1 have been shown to have a positive, rather than negative, input in an alternative signaling cascade mediated by MOM-2/Wnt and WRM-1/ beta-catenin. Furthermore, an Axin-related protein has not been found by sequence based homology searches. In the Q neuroblast lineage, a difference in sensitivity to EGL-20 restricts the activation of
mab-5 to QL and its daughter cells (QL.d - left side). This left/right asymmetric expression of
mab-5 is responsible for the difference in migration of the Q cells: the QL.d migrate towards the posterior, whereas on the right side, the QR.d migrate in the default anterior direction.
pry-1 has been described by Maloof et al. to negatively regulate the EGL-20/BAR-1 mediated control of
mab-5 in the Q lineage1.
pry-1 loss-of function mutants show Wnt activation phenotypes, such as ectopic expression of
mab-5 in the QR lineage and subsequent posterior migration of the QR daughter cells. To better understand the mechanism of BAR-1 regulation by the EGL-20 signal, we have characterized
pry-1. We find that
pry-1 encodes a protein that contains a DIX and RGS domain and is distantly related to Axin. Despite this sequence divergence, we show that PRY-1 is a functional Axin homologue. As predicted for a potential Axin, PRY-1 physically interacts with BAR-1/ beta-catenin, SSG-1/GSK3, APR-1 and MIG-5/Dsh. Furthermore,
pry-1 functions downstream of
egl-20 and
mig-5 but upstream of
bar-1,
pop-1 and
mab-5. Overexpression of pry- 1 inhibits
mab-5 expression in QL and results in anterior migration of its daughter cells, a phenotype similar to the
egl-20/Wnt loss-of function phenotype. Finally, we show that overexpression of
pry-1 rescues the zebrafish axin1 mutation masterblind, demonstrating the functional similarity of PRY-1 and Axin. In addition, we find that overexpression of
sgg-1 also results in a Wnt loss-of function phenotype in the Q lineage and inhibits the Wnt-1 induced activation of a Tcf-reporter gene in vertebrate cells. Recently,
apr-1(RNAi) has been shown to strongly enhance the
pry-1 vulval phenotype2 (a similar pathway controls the expression of
lin-39 in the vulva precursor cells). These data suggest an additional negative role for SGG-1 and APR- 1 in the BAR-1 mediated Wnt pathway. We therefore propose that PRY-1 is a functional Axin homologue and that a highly divergent destruction complex consisting of PRY-1, SGG-1 and APR-1 regulates BAR-1 levels in C. elegans.