Pleckstrin homology domains of phospholipase C-γ1 directly interact with β-tubulin for activation of phospholipase C-γ1 and reciprocal modulation of β-tubulin function in microtubule assembly

Phosphoinositide-specific phospholipase C-γ1 (PLC-γ1) has two pleckstrin homology (PH) domains, an N-terminal domain and a split PH domain. Here we show that pull down of NIH3T3 cell extracts with PLC-γ1 PH domain-glutathione S-transferase fusion proteins, followed by matrix-assisted laser desorption ionization-time of flight-mass spectrometry, identified β-tubulin as a binding protein of both PLC-γ1 PH domains. Tubulin is a main component of microtubules and mitotic spindle fibers, which are composed of α- and β-tubulin heterodimers in all eukaryotic cells. PLC-γ1 and β-tubulin colocalized in the perinuclear region in COS-7 cells and cotranslocated to the plasma membrane upon agonist stimulation. Membrane-targeted translocation of depolymerized tubulin by agonist stimulation was also supported by immunoprecipitation analyses. The phosphatidylinositol 4,5-bisphosphate (PIP₂) hydrolyzing activity of PLC-γ1 was substantially increased in the presence of purified tubulin in vitro, whereas the activity was not promoted by bovine serum albumin, suggesting that β-tubulin activates PLC-γ1. Furthermore, indirect immunofluorescent microscopy showed that PLC-γ1 was highly concentrated in mitotic spindle fibers, suggesting that PLC-γ1 is involved in spindle fiber formation. The effect of PLC-γ1 in microtubule formation was assessed by overexpression and silencing PLC-γ1 in COS-7 cells, which resulted in altered microtubule dynamics in vivo. Cells overexpressing PLC-γ1 showed higher microtubule densities than controls, whereas PLC-γ1 silencing with small interfering RNAs led to decreased microtubule network densities as compared with control cells. Taken together, our results suggest that PLC-γ1 and β-tubulin transmodulate each other, i.e. that PLC-γ1 modulates microtubule assembly by β-tubulin, and β-tubulin promotes PLC-γ1 activity.