Role of STRAP in regulating GSK3ß function and Notch3 stabilization.

ABSTRACT

Glycogen synthase kinase 3ß (GSK3ß) can regulate a broad range of cellular processes in a variety of cell types and tissues through its ability to phosphorylate its substrates in a cell- and time-specific manner. Although it is known that Axin and presenilin help to recruit ß-catenin/Smad3 and tau protein to GSK3ß, respectively, it is not clear how many of the other GSK3ß substrates are recruited to it. Here, we have established the binding of GSK3ß with a novel scaffold protein, STRAP, through its WD40 domains. In a new finding, we have observed that STRAP, GSK3ß and Axin form a ternary complex together. We show for the first time that intracellular fragment of Notch3 (ICN3) binds with GSK3ß through the ankyrin repeat domain. This binding between STRAP and GSK3ß is reduced by small-molecule inhibitors of GSK3ß. Further studies revealed that STRAP also binds ICN3 through the ankyrin repeat region, and this binding is enhanced in a proteasomal inhibition-dependent manner. In vivo ubiquitination studies indicate that STRAP reduces ubiquitination of ICN3, suggesting a role of STRAP in stabilizing ICN3. This is supported by the fact that STRAP and Notch3 are co-upregulated and co-localized in 59% of non-small cell lung cancers, as observed in an immunohistochemical staining of tissue microarrays. These results provide a potential mechanism by which STRAP regulates GSK3ß function and Notch3 stabilization and further support the oncogenic functions of STRAP.