Requirement of Prorenin Receptor and Vacuolar H + -ATPase–Mediated Acidification for Wnt Signaling

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Abstract
Of Wnt, Prorenin Receptor, and V-ATPase: The Wnt protein binds to receptors at the cell surface and regulates signaling pathways that control a wide range of critical biological processes from stem cell differentiation to generation of cancer. In a screen for components required for Wnt signaling, Cruciat et al. (p. 459 ) discovered an unexpected partner, the prorenin receptor (PRR). PRR bound to the Wnt receptor proteins Fz8 and LRP6. The PRR protein could interact with the receptors and promote Wnt signaling without its cytoplasmic domain through which it initiates signaling in response to the prorenin protein. Its role in Wnt signaling appears to be rather different. The PRR binds to the vacuolar H + –adenosine triphosphatase (V-ATPase), a proton pump that can influence endocytosis by acidification of vesicles. The V-ATPase was also necessary for phosphorylation of LRP6 and Wnt. Thus, PRR may link the V-ATPase to the Wnt receptor protein LRP6, allowing acidification in the vicinity of the activated receptor, which appears to be necessary for phosphorylation of LRP6 and subsequent signaling.