Activation of phospholipase C isozymes by G protein beta gamma subunits

Mechanisms of cellular information processing

Living cells in multicellular organisms are in simultaneous contact with many regulatory factors such as hormones or neurotransmitters. Many of these factors vary with time in their local concentrations, owing to pulsatile release or production. Therefore, complex patterns of signaling factors act on each living cell in vivo, stimulating or inhibiting second-messenger pathways with potentially complex dynamics. These intracellular pathways do not operate independently but are extensively interconnected, creating complex networks and patterns of intracellular signals that combine to determine the cell's response. The potential significance of cross-signaling between second-messenger pathways and of dynamic stimulation of receptors for cellular information processing in physiology and pathophysiology are discussed.

Mutational analysis of phospholipase C-beta 2. Identification of regions required for membrane association and stimulation by guanine-nucleotide-binding protein beta gamma subunits

Members of the beta isozyme subfamily of the phosphoinositide-specific phospholipases C (PLC beta) have recently been shown to be stimulated by both guanine-nucleotide-binding protein alpha and beta gamma subunits. The alpha subunits of the Gq class activate PLC beta isozymes in the order of PLC beta 1 > or = PLC beta 3 >> PLC beta 2, which is different from the order of PLC beta 3 > PLC beta 2 > PLC beta 1 for beta gamma subunit stimulation. The C-terminal region of PLC beta 1, in particular the sequence between Thr903 and Leu1142, has been shown to be involved in interacting with activated alpha q subunits and to contain a region required for efficient membrane association of PLC beta 1 [Park, D., Jhon, D.-Y., Lee, C.-W., Ryu, S. H. & Rhee, S. G. (1993) J. Biol. Chem. 268, 3710-3714, and Wu, D., Jiang, H., Katz, A. & Simon, M. I. (1993) J. Biol. Chem. 268, 3704-3709]. To examine the structure-function relationships of a PLC beta isozyme highly sensitive to beta gamma subunit stimulation, we have altered the cDNA of PLC beta 2 by site-directed mutagenesis and have examined the effects of these structural alterations on the functional properties of the mutant polypeptides. The results show that the C-terminal region of PLC beta 2 downstream of Phe818, which corresponds to Tyr816 of PLC beta 1, contains a region essential for membrane association, but is required neither for the interaction of PLC beta 2 with Ca2+ and the phospholipid substrate, nor for beta gamma subunit stimulation of PLC beta 2. These data suggest that PLC beta isozymes are activated by alpha q and beta gamma subunits via distinct domains.

A G-protein-coupled 130 kDa phospholipase C isozyme, PLC-beta 4, from the particulate fraction of bovine cerebellum

A 130 kDa PLC isozyme was purified from the particulate fraction of bovine cerebellum. This PLC was recognized by a polyclonal antiserum generated against the purified 97 kDa PLC-beta 4. Reconstitution of the purified 130 kDa PLC with the membranes of C6 Bu-1 cells in the presence of GTP gamma S or AlF4- resulted in PLC activation as well as the association of PLC with the membranes. Both the association and activation were revoked when the membrane was washed with 2 M KCl. The 97 kDa PLC-beta 4 did not associate with membranes. These data suggest that the 130 kDa PLC is the intact form of PLC-beta 4 the activity of which is likely to be regulated by a G-protein on the membrane.

Stable changes in expression or activation of G protein alpha i or alpha q subunits affect the expression of both beta 1 and beta 2 subunits

G proteins consist of three subunits: alpha, beta and gamma. Four beta subunits have been cloned: beta 1 and beta 4 (36 kDa), and beta 2 and beta 3 (35 kDa). We studied endogenous beta subunits in mouse NIH 3T3 fibroblasts stably expressing high levels of G protein alpha subunits after transfection with cDNAs encoding alpha i1, alpha i2, alpha i3 and alpha q. Immunoblots showed that NIH 3T3 cells express beta 36 and beta 35 subunits; in these cells, beta 35 subunits are four times more abundant than beta 36 subunits. We could detect beta 1 and beta 2 mRNA, but neither beta 3 nor beta 4 mRNA. We found that a stable increase in expression of wild-type alpha i1, alpha i2, alpha i3 or alpha q subunits is always accompanied by an increase in beta 1 and beta 2 mRNA and protein levels. There was no evidence of selectivity for an increase in beta 1 rather than beta 2 subunits depending on the type of alpha subunit overexpressed. However, constitutive activation or inactivation of alpha subunits induced specific changes in beta subunits. Expression of constitutively inactivated alpha i2 subunits was accompanied by an increase in mRNA and protein levels of both beta subunits. In contrast, cells expressing constitutively activated alpha i2 subunits did not show any change in the amount of beta proteins expressed in membranes, despite a significant increase in beta 1 and beta 2 mRNA. We conclude that stable changes in the levels of expression or degree of activation of G alpha subunits affect the level of expression, and possibly the turn-over, of beta subunits, without selectivity among beta 1 and beta 2 subunits.

The human muscarinic M1 acetylcholine receptor, when express in CHO cells, activates and downregulates both Gq alpha and G11 alpha equally and non-selectively

CHO cells express both of the phosphoinositidase C-linked G-proteins Gq and G11. G11 alpha is some 2.5-fold more highly expressed than Gq alpha in membranes of these cells. Following transfection and stable expression of CHO cells with DNA encoding the human muscarinic M1 acetylcholine (HM1) receptor, chronic treatment of the cells with the cholinergic agonist carbachol resulted in down-regulation of membrane levels of both Gq alpha and G11 alpha. Dose-response curves to carbachol produced identical EC50 values for agonist-induced down-regulation of the two G-proteins and both were down-regulated with the same time course. These data indicate that the HM1 receptor interacts with the activates both Gq alpha and G11 alpha equivalently and non-selectively in a whole cell system in which the receptor has access to both G-proteins.