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2014 ; 12
(ä): 44
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Molecular dissection of Wnt3a-Frizzled8 interaction reveals essential and
modulatory determinants of Wnt signaling activity
#MMPMID24885675
Kumar S
; ?igman M
; Patel TR
; Trageser B
; Gross JC
; Rahm K
; Boutros M
; Gradl D
; Steinbeisser H
; Holstein T
; Stetefeld J
; Özbek S
BMC Biol
2014[May]; 12
(ä): 44
PMID24885675
show ga
BACKGROUND: Wnt proteins are a family of secreted signaling molecules that
regulate key developmental processes in metazoans. The molecular basis of Wnt
binding to Frizzled and LRP5/6 co-receptors has long been unknown due to the lack
of structural data on Wnt ligands. Only recently, the crystal structure of the
Wnt8-Frizzled8-cysteine-rich-domain (CRD) complex was solved, but the
significance of interaction sites that influence Wnt signaling has not been
assessed. RESULTS: Here, we present an extensive structure-function analysis of
mouse Wnt3a in vitro and in vivo. We provide evidence for the essential role of
serine 209, glycine 210 (site 1) and tryptophan 333 (site 2) in Fz binding.
Importantly, we discovered that valine 337 in the site 2 binding loop is critical
for signaling without contributing to binding. Mutations in the presumptive
second CRD binding site (site 3) partly abolished Wnt binding. Intriguingly, most
site 3 mutations increased Wnt signaling, probably by inhibiting Wnt-CRD
oligomerization. In accordance, increasing amounts of soluble Frizzled8-CRD
protein modulated Wnt3a signaling in a biphasic manner. CONCLUSIONS: We propose a
concentration-dependent switch in Wnt-CRD complex formation from an inactive
aggregation state to an activated high mobility state as a possible modulatory
mechanism in Wnt signaling gradients.