RPS3 phosphorylation in response to NF-κB activation
To test whether RPS3 is phosphorylated during NF-κB activation, we performed 32P-labeling experiments in tumor necrosis factor (TNF)-stimulated HEK 293T cells. While RPS3 was scarcely phosphorylated in unstimulated cells, we observed a marked increase in 32P-incorporation after TNF stimulation despite no increase in RPS3 protein (Fig. 1a). To determine which RPS3 residues were phosphorylated, we immunoprecipitated RPS3 from either resting or stimulated cells and performed immunoblotting with phosphorylation-specific antibodies. Both TNF and phorbol myristate acetate/ionomycin (PMA+I) stimulated rapid phosphorylation and degradaion of IκBα within 5 min which was accompanied by RPS3 phosphorylation on serine residues (Fig. 1b and data not shown), similar to the in vivo labeling. We did not detect tyrosine- or threonine-phosphorylation of RPS3 (Fig. 1b).