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Biol. Pharm. Bull. 35(12) 2103–2111 (2012)

December 2012

2103

Review

The Inhibitory G Protein Gi Identified as Pertussis
Toxin-Catalyzed ADP-Ribosylation
Toshiaki Katada
Department of Physiological Chemistry, Graduate School of Pharmaceutical Sciences,
The University of Tokyo;
7–3–1 Hongo, Bunkyo-ku, Tokyo 113–0033, Japan.
Received August 20, 2012
Pertussis toxin (PTX) producedby Bordetella pertussis was first introduced by Ui and his colleagues
in research on signal transduction under the name islet-activating protein in 1979, when the mechanism
of toxin-induced stimulation of insulin release from pancreatic islets was reported in the rat. The stimulatory effect of PTX in vivo results from the blockage of α2-adrenergic receptor-mediated inhibition of insulin
release.The receptor-induced inhibition of cAMP formation was also abolished in pancreatic islets isolated
from PTX-treated rats, suggesting that the toxin caused uncoupling of adenylyl cyclase inhibition from re ceptor stimulation. The action of PTX on isolated membranes required a cytosolic factor, nicotinamide adenine dinucleotide (NAD), and the uncoupling induced by PTX was shown to be due to thetoxin-catalyzed
ADP-ribosylation of a 41-kDa protein with NAD as another substrate. The 41-kDa PTX substrate was soon
identified and purified as the α-subunit of the inhibitory G protein that transmits an inhibitory signal from
membrane receptors to adenylyl cyclase. After demonstration of the molecular mechanism of PTX, the toxin
was widely utilized as a probe for identifying and analyzing majorαβγ-trimeric G proteins. Thus, PTX-sensitive G proteins appeared to carry positive and negative signals from many membrane receptors to a variety
of effectors other than adenylyl cyclase.
Key words
duction

1.

G protein; pertussis toxin; ADP-ribosylation; receptor; guanosine 5 -triphosphate; signal trans-

INTRODUCTION

In many intracellular signaling pathways, guanosine 5 -triphosphate(GTP)-binding proteins (G proteins), which cycle
between the two different conformations of GTP-bound active and guanosine 5 -diphosphate (GDP)-bound inactive
states, play important roles as a “molecular switch.” There
are three major G-protein families: 1) translational initiation,
elongation, and termination factors that are involved in protein
synthesis and mRNA degradation; 2)αβγ-trimeric G proteins
that associate with membrane receptors for various hormones
and transmit the extracellular signals into intracellular mechanisms; and 3) small GTPases involved in the regulation of cell
g rowth and differentiation, intracellular vesicle trafficking,
and cell shape and adhesion. During the last four decades, I
had the opportunity to study all three families of G proteins.
In thisreview, I would like to present findings from my earlier
studies leading to the discovery of the αβγ-trimeric G protein
G i.
Many hormones (first messengers) bind to their specific receptors and generate intracellular signals (second messengers),
such as cAMP. In the late 1970s and early 1980s, the most
widely known signaling pathway was the cAMP-stimulation
pathway induced by the “membranereceptor–G protein (Gs)–
adenylyl cyclase” cascade. I found that epinephrine could
conversely inhibit cAMP formation via the α2-adrenergic receptor and that pertussis toxin (PTX) completely abolished the
inhibitory action of epinephrine. The action of PTX was soon
shown to be due to nicotinamide adenine dinucleotide (NAD)dependent ADP-ribosylation of a new G protein that was
different from Gs. Thenew G protein, which appeared to be
involved in the inhibition of adenylyl cyclase, was henceforth

referred to as Gi. Interestingly, all the membrane receptors
lost their ability to inhibit cAMP formation when Gi had been
ADP-ribosylated by PTX. Thus, Gi was discovered to be the
target protein of PTX, and this bacterial toxin was widely
used as a probe to analyze the involvement of...