TUBULIN, G(Q), AND PHOSPHATIDYLINOSITOL 4,5-BISPHOSPHATE INTERACT TO REGULATE PHOSPHOLIPASE C-BETA(1) SIGNALING

The cytoskeletal protein, tubulin, has been shown to regulate adenylyl cyclase activity through its interaction with the specific G protein alpha subunits, G alpha(s) or G alpha(11). Tubulin activates these G p roteins by transferring GTP and stabilizing the active nucleotide-boun d G alpha conformation. To study the possibility of tubulin involvemen t in G alpha(q)-mediated phospholipase C beta(1) (PLC beta(1)) signali ng, the m(1) muscarinic receptor, G alpha(q), and PLC beta(1) were exp ressed in Sf9 cells. A unique ability of tubulin to regulate PLC beta( 1) was observed. Low concentrations of tubulin, with guanine nucleotid e bound, activated PLC beta(1), whereas higher concentrations inhibite d the enzyme. Interaction of tubulin with both G alpha(q) and PLC beta (1), accompanied by guanine nucleotide transfer from tubulin to G alph a(q), is suggested as a mechanism for the enzyme activation. The PLC b eta(1) substrate, phosphatidylinositol 4,5-bisphosphate, bound to tubu lin and prevented microtubule assembly. This observation suggested a m echanism for the inhibition of PLC beta(1) by tubulin, since high tubu lin concentrations might prevent the access of PLC beta(1) to its subs trate. Activation of m(1) muscarinic receptors by carbachol relaxed th is inhibition, probably by increasing the affinity of G alpha(q) for t ubulin. Involvement of tubulin in the articulation between PLC beta(1) signaling and microtubule assembly might prove important for the intr acellular governing of a broad range of cellular events.