Ski antagonizes BMP signaling in Xenopus explants.
(A) Ski blocks BMP2 activity. Explants were cultured
until stage 10.5 and analyzed for Xbra expression by RT-PCR.
Lanes 1 and 2, whole embryo control and no RT (RT−); lane 3,
uninjected explants; lanes 4 and 5, explants expressing AB2; lanes
6–8, explants expressing AB2 with increasing amounts of ski
mRNA; lanes 9–11, explants expressing ski mRNA
only. (B) Ski fails to neuralize in the presence of high
levels of the constitutive active BMPRI (Alk3*). Explants were analyzed
at stage 20 for NCAM expression. Lanes 3–5, explants
expressing AB2 mRNA only; lane 6–8, explants expressing
ski and increasing amounts of Alk3* as indicated.
Alk3*, (Q233D)Alk3. (C) Ski blocks the activity of
Smad1. Lane 3, uninjected sample; lane 4, Smad1 stimulates the
expression of the ventral markers, Xvex-1 and
Xhox-3; lanes 5–7, explants expressing Smad1
mRNA in combination with increasing amounts of ski
mRNA; Lane 8, ski alone.