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Fig. 1. Models for Wnt signal production and reception. (A) Wnt
signal production. Wnt is palmitoylated (red rectangle) by Porcupine (Porc) in
the lumen of the endoplasmic reticulum (ER). Upon reaching the Golgi, Wntless
(also known as Evenness interrupted and Sprinter, and as MIG-14 in C.
elegans; Wls, orange) associates with Wnt in vesicles bound for secretion
(arrows), leading to extracellular association of Wnt with lipoprotein
particles (Lp). The retromer complex recycles Wls at the plasma membrane back
to the Golgi. (B) A two-step model for Wnt signal reception. In the
absence of a Wnt signal, the labile ß-catenin molecule is phosphorylated
by a complex consisting of Axin, Apc, and the kinases CK1 and GSK3ß, and
is then rapidly degraded. Signaling is initiated (left) by association of Wnt
with its co-receptors Lrp5/6 and Fz at the plasma membrane, leading to the
Dsh-dependent partial inhibition of ß-catenin degradation and its
translocation through the nuclear membrane. In the nucleus, ß-catenin
activates target gene transcription by associating with Tcf. Signal
amplification (right) occurs via GSK3- and CK1-dependent phosphorylation (P)
of Lrp5/6, leading to further recruitment of Axin and Dsh oligomers (n) to
form the `Wnt signalosome', causing increased accumulation of nuclear
ß-catenin and further upregulation of target gene transcription. See text
for references.
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-tubulin (green), and histone (blue). Sister cells are connected
by white bars. Notice that ß-catenin (red) is present at high levels in
the nuclei of vegetal-pole sister cells (arrowheads, magenta color), but at
low levels in the nuclei of animal-pole sisters. Image courtesy of Stephan
Schneider. See text for references. Scale bars: 
150 µm in A for A,B; 2
µm in C.