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β-catenin stabilization results in its higher nuclear levels, but how β-catenin is shuttled to and retained in the nucleus is not well understood Earlier studies suggested that β-catenin enters the nucleus in an NLS (nuclear localization signal)- and importin-independent fashion by interacting directly with nuclear pore proteins 。 β-atenin also exits the nucleus via export involving APC 、Axin,and RanBP3 (Ran binding protein 3), which binds to β-catenin in a Ran-GTP dependent manner . Live cell imaging suggests that while Axin and APC can enrich β-catenin in the cytoplasm and TCF and β-catenin co-activators (BCL9 and Pygopus, see below) increase! l, t) l% y% i5 [) A8 r% }: b
nuclear β-catenin, they do not accelerate the export or import rate of β-catenin, thereby arguing" v) a" c- ]. R1 n: f/ r
for their roles in β-catenin retention rather than shuttling . Thus β-catenin nuclear and cytoplasmic partitioning is likely the dynamic sum of both shuttling and retention between the two compartments via multiple mechanisms.A recent study argues that Wnt-induced β-catenin stabilization is not sufficient for its nuclear
5 ]( {; G1 b% g9 M4 x9 X/ laccumulation, but Wnt activation of the Rac1 GTPase is required in parallel . Specifically Rac1, JNK2 (Jun N-terminal kinase 2) and β-catenin form a cytoplasmic complex, and JNK2 phosphorylates β-catenin (at serines 191 and 605) and promotes it nuclear translocation。
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