Identification of ribosomal protein S3a as a candidate for a novel PI 3-kinase target in the nucleus.

Phosphatidylinositol 3,4,5-trisphosphate (PIP(3)) is an important lipid second messenger that mediates various cell responses. We have searched for the nuclear PIP(3) binding proteins using PIP(3) analogue beads. A 33 kD protein was detected in this method, which was identified as ribosomal protein S3a by the mass spectrometric analysis. The recombinant S3a protein bound specifically to PIP(3). S3a localized not only in the cytosol but also in the nucleus. Interestingly, not cytosolic but nuclear S3a bound to PIP(3), suggesting different roles of S3a in the cytosol and the nucleus.