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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) increase0 S5 m: f1 H) H- r0 G1 g
nuclear β-catenin, they do not accelerate the export or import rate of β-catenin, thereby arguing& k3 \8 f. X; i1 P+ ]6 W' |
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
; ]2 J" J! L- Vaccumulation, 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。- n' Y9 _7 b# [7 Y" Z' s
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