Regulation of canonical Wnt signalling by ciliary protein MKS1 and Ubiquitin Proteasome System component UBE2E1.

A functional primary cilium is a crucial cell appendage which is essential for normal, regulated signalling, and loss of the primary cilium is implicated in a suite of severe developmental conditions known as ciliopathies. The mechanisms of signal regulation by the cilium remain unclear. Previous studies have suggested links between the primary cilium/basal body, the ubiquitin proteasome system (UPS) and Wnt signalling. Here we provide further mechanistic insights into these processes in vivo by crossing the Mks1-/- knockout mouse with the UbG76V-GFP reporter line. We demonstrate in vivo that MKS1 is essential for normal proteasomal processing of ubiquitinated proteins, but that this only manifests as accumulation of ubiquitinated proteins when the proteasome is inhibited. We show that an increase in proteasomal enzymatic activity and Wnt signalling de-regulation in the absence of MKS1. Yeast 2-hybrid demonstrate that the UPS component UBE2E1, an E2 ubiquitin-conjugating enzyme which polyubiquitinates β-catenin, interacts with MKS1. Levels of UBE2E1 and MKS1 are co-dependent, and loss of UBE2E1 recapitulates the ciliary and Wnt signalling phenotypes observed during loss of MKS1, suggesting a functional association between the two proteins. We suggest that MKS1 regulates UBE2E1 and other UPS components at the base of the cilium, which leads to proteasomal and canonical Wnt signalling dysregulation. These findings provide further mechanistic detail of the interaction between the basal body and the UPS in regulating signal transduction through β-catenin, and confirm that the UPS plays a central role in the molecular pathogenesis of ciliopathies.