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Fig. 3. Sequences within the N terminus of Slug are responsible for its
ubiquitin-mediated proteasomal degradation. (A) Schematic
illustrating the mutant constructs used in these experiments. (B)
Western blot demonstrating that the C-terminal zinc fingers of Slug are highly
stable, but the stability of the N terminus is comparable to that of the
intact protein. (C) Western blot analysis was used to monitor the
relative stability of Slug with a series of Slug deletion mutants. When
compared to the full-length protein, the stability of
P38 was unchanged
but the
H64 mutant was greatly stabilized. (D) Western blot
comparing the stability of Sox10 and a Slug N-Sox10 fusion protein. Addition
of amino acids M1-L63 of Slug to Sox10 was sufficient to render it unstable
(double bands represent a partial degradation product). (E)
Poly-ubiquitinated forms of Slug were immunoprecipitated from lysates of
embryos co-injected with tagged forms of Slug and ubiquitin. Slug is indicated
by an arrow. In all blots an asterisk indicates IgG background band.
(F) Embryos were co-injected with a mutant form of Ub
(UbK48R) and either Slug,
P38, or Slug-C and collected for
western blot analysis. Whereas Slug and the unstable deletion mutant
P38 both incorporate UbK48R (arrow), the stable C-terminal
zinc fingers do not. (G) Western blot demonstrating the accumulation of
higher molecular mass forms of wild-type Slug, but not the Slug-C or Slug
P31-H64 deletion mutants, in response to treatment with the proteasomal
inhibitor, MG132.