Facilitation of signal onset and termination by adenylyl cyclase

Cysteine 3 is not the site of in vitro palmitoylation on G(salpha)

Several studies have examined the role of palmitoylation of G protein alpha subunits by nonenzymatic in vitro acylation using palmitoyl-CoA. Here, we investigated nonenzymatic palmitoylation of purified G(salpha) in vitro. GDP-bound G(salpha) was stoichiometrically autoacylated on cysteine residue(s) with micromolar concentrations of palmitoyl-CoA. The acylation led to a complete loss of steady-state GTPase activity and GTPgammaS binding to G(salpha). Mutation of Cys 3 to Ala in G(salpha) did not prevent either palmitoylation or its consequent functional alterations. However, stoichiometric palmitoylation of His(6)-G(salpha) did not alter its GTPase activity or GTPgammaS binding. Isoelectric focusing of tryptic peptides from autoacylated wild type, His(6)-tagged, and C3A mutant of G(salpha) showed that Cys 160 is the site of in vitro palmitoylation. Therefore, we conclude that in vitro palmitoylation of G(salpha) occurs on Cys 160 and this modification decreases the ability of the protein to exchange GTP for GDP; N-terminus elongation of G(salpha) prevents this latter effect without altering palmitoylation.

Regulation of adenylyl cyclase in the central nervous system

Adenylyl cyclases (ACs) are a family of enzymes that synthesize one of the major second messengers, cAMP, upon stimulation. Since the report of the first adenylyl cyclase (AC) gene in 1989, tremendous efforts have been devoted to identifying and characterizing more AC isozymes. In the past decade, significant knowledge regarding the basic structure, tissue distribution, and regulation of AC isozymes has been accumulated. Because members of the AC superfamily are tightly controlled by various signals, one of the most important impacts of these AC isozymes is their contribution to the complexity and fine-tuning of cellular signalling, especially in the central nervous system (CNS) where multiple signals constantly occur. This review focuses on recent progress toward understanding the physiological roles of ACs in the CNS.

Rapid GTP binding and hydrolysis by G(q) promoted by receptor and GTPase-activating proteins

Receptor-promoted GTP binding and GTPase-activating protein (GAP)-promoted GTP hydrolysis determine the onset and termination of G protein signaling; they coordinately control signal amplitude. The mechanisms whereby cells independently regulate signal kinetics and signal amplitude are therefore central to understanding G protein function. We have used quench-flow kinetic methods to measure the rates of the individual reactions of the agonist-stimulated GTPase cycle for G(q) during steady-state signaling. G(q) and m1 muscarinic cholinergic receptor were co-reconstituted into proteoliposomes with one of two GAPs: phospholipase C (PLC)-beta1, the major G(q)-regulated effector protein, and RGS4, a GAP commonly thought to be an inhibitor of G(q) signaling. In this system, the rate constant for GAP-stimulated hydrolysis of Galpha(q)-bound GTP at 30 degrees C was 9-12 s(-1) for PLC-beta1 and 22-27 s(-1) for RGS4. These rates are 1,000- to 2,000-fold faster than in the absence of a GAP and far faster than measured previously. G(q) can thus hydrolyze bound GTP with deactivation half-times of 25-75 ms at 30 degrees C, commensurate with physiological rates of signal termination. GDP/GTP exchange, which reactivates G(q), was the principal rate-limiting step for the GTPase cycle and was also faster than previously thought. At physiological concentrations of GTP, exchange was limited by the rate of dissociation of GDP from the receptor-G(q) complex, with a maximal rate of 1.8 s(-1) at 30 degrees C. Comparison of activation and deactivation rates help explain how GDP/GTP exchange balance rapid GTP hydrolysis to maintain steady-state signal amplitude.