Identification of a novel binding partner of phospholipase cß1: translin-associated factor X.

ABSTRACT

Mammalian phospholipase Cß1 (PLCß1) is activated by the ubiquitous Gα(q) family of G proteins on the surface of the inner leaflet of plasma membrane where it catalyzes the hydrolysis of phosphatidylinositol 4,5 bisphosphate. In general, PLCß1 is mainly localized on the cytosolic plasma membrane surface, although a substantial fraction is also found in the cytosol and, under some conditions, in the nucleus. The factors that localize PLCß1in these other compartments are unknown. Here, we identified a novel binding partner, translin-associated factor X (TRAX). TRAX is a cytosolic protein that can transit into the nucleus. In purified form, PLCß1 binds strongly to TRAX with an affinity that is only ten-fold weaker than its affinity for its functional partner, Gα(q). In solution, TRAX has little effect on the membrane association or the catalytic activity of PLCß1. However, TRAX directly competes with Gα(q) for PLCß1 binding, and excess TRAX reverses Gα(q) activation of PLCß1. In C6 glia cells, endogenous PLCß1 and TRAX colocalize in the cytosol and the nucleus, but not on the plasma membrane where TRAX is absent. In Neuro2A cells expressing enhanced yellow and cyano fluorescent proteins (i.e., eYFP- PLCß1 and eCFP-TRAX), Förster resonance energy transfer (FRET) is observed mostly in the cytosol and a small amount is seen in the nucleus. FRET does not occur at the plasma membrane where TRAX is not found. Our studies show that TRAX, localized in the cytosol and nucleus, competes with plasma-membrane bound Gα(q) for PLCß1 binding thus stabilizing PLCß1 in other cellular compartments.