Tubulin, Gq, and Phosphatidylinositol 4,5-Bisphosphate Interact to Regulate Phospholipase Cβ1 Signaling

Abstract

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein α subunits, Gαs or Gαi1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Gα conformation. To study the possibility of tubulin involvement in Gαq-mediated phospholipase Cβ1 (PLCβ1) signaling, the m1 muscarinic receptor, Gαq, and PLCβ1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCβ1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCβ1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Gαq and PLCβ1, accompanied by guanine nucleotide transfer from tubulin to Gαq, is suggested as a mechanism for the enzyme activation. The PLCβ1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCβ1 by tubulin, since high tubulin concentrations might prevent the access of PLCβ1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Gαq for tubulin. Involvement of tubulin in the articulation between PLCβ1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events. The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein α subunits, Gαs or Gαi1. Tubulin activates these G proteins by transferring GTP and stabilizing the active nucleotide-bound Gα conformation. To study the possibility of tubulin involvement in Gαq-mediated phospholipase Cβ1 (PLCβ1) signaling, the m1 muscarinic receptor, Gαq, and PLCβ1 were expressed in Sf9 cells. A unique ability of tubulin to regulate PLCβ1 was observed. Low concentrations of tubulin, with guanine nucleotide bound, activated PLCβ1, whereas higher concentrations inhibited the enzyme. Interaction of tubulin with both Gαq and PLCβ1, accompanied by guanine nucleotide transfer from tubulin to Gαq, is suggested as a mechanism for the enzyme activation. The PLCβ1 substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubulin and prevented microtubule assembly. This observation suggested a mechanism for the inhibition of PLCβ1 by tubulin, since high tubulin concentrations might prevent the access of PLCβ1 to its substrate. Activation of m1 muscarinic receptors by carbachol relaxed this inhibition, probably by increasing the affinity of Gαq for tubulin. Involvement of tubulin in the articulation between PLCβ1 signaling and microtubule assembly might prove important for the intracellular governing of a broad range of cellular events.