1. Thyroid-stimulating Hormone/cAMP and Glycogen Synthase Kinase 3β Elicit Opposing Effects on Rap1GAP Stability.
- Author
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Tsygankova, Oxana M., Feshchenko, Elena, Klein, Peter S., and Meinkoth, Judy L.
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GTPASE-activating protein , *HORMONES , *CYCLIC adenylic acid , *PROTEINS , *UBIQUITIN , *THYROID gland , *GLYCOGEN - Abstract
Beyond regulating Rap activity, little is known regarding the regulation and function of the Rap GTPase-activating protein Rap1GAP. Tuberin and E6TP1 protein levels are tightly regulated through ubiquitin-mediated proteolysis. A role for these RapGAPs, along with SPA-1, as tumor suppressors has been demonstrated. Whether RaplGAP performs a similar role was investigated. We now report that Rap1GAP protein levels are dynamically regulated in thyroid-stimulating hormone (TSH)-dependent thyroid cells. Upon TSH withdrawal, Rap1GAP undergoes a net increase in phosphorylation followed by proteasome-mediated degradation. Sequence analysis identified two putative destruction boxes in the Rap1GAP C-terminal domain. Glycogen synthase kinase 3β (GSK3β) phosphorylated Rap1GAP immunoprecipitated from thyroid cells, and GSK3β inhibitors prevented phosphorylation and degradation of endogenous Rap1GAP. Co-expression of GSK3β and Rap1GAP in human embryonic kidney 293 cells stimulated proteasome-dependent Rap1GAP turnover. Mutational analysis established a role for serine 525 in the regulation of Rap1GAP stability. Overexpression of Rap1GAP in thyroid cells impaired TSH/cAMPstimulated p70S6 kinase activity and cell proliferation. These data are the first to show that Rap1GAP protein levels are tightly regulated and are the first to support a role for Rap1GAP as a tumor suppressor. [ABSTRACT FROM AUTHOR]
- Published
- 2004
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