G(i/o) protein-dependent and -independent actions of Pertussis Toxin (PTX)

Abstract

Pertussis toxin (PTX) is a typical A-B toxin. The A-protomer (S1 subunit) exhibits ADP-ribosyltransferase activity. The B-oligomer consists of four subunits (S2 to S5) and binds extracellular molecules that allow the toxin to enter the cells. The A-protomer ADP-ribosylates the α subunits of heterotrimeric G(i/o) proteins, resulting in the receptors being uncoupled from the G(i/o) proteins. The B-oligomer binds proteins expressed on the cell surface, such as Toll-like receptor 4, and activates an intracellular signal transduction cascade. Thus, PTX modifies cellular responses by at least two different signaling pathways; ADP-ribosylation of the Gα(i/o) proteins by the A-protomer (G(i/o) protein-dependent action) and the interaction of the B-oligomer with cell surface proteins (G(i/o) protein-independent action).

Keywords: A-protomer; ADP-ribosylation; B-oligomer; G protein-coupled receptor; Gi/o-dependent; Gi/o-independent; Toll-like receptor 4; heterotrimeric G protein; pertussis toxin.

Figure 1

Pertussis toxin (PTX) structural organization. PTX contains five different subunits that are arranged in a typical A-B structure. The A-protomer contains an enzymatically active S1 subunit that is on the top of B-oligomer. The B-oligomer is composed of two dimers, S2-S4 and S3-S4 dimers, which are held together by the S5 subunit.

Figure 2

Schematic diagram of the ADP-ribosylation of α subunit of heterotrimeric G_i/o protein by pertussis toxin (PTX). PTX catalyzes the cleavage of the C-N bond between a carbon atom of ribose and a nitrogen atom of nicotinamide, and transfers the ADP-ribosyl moiety to an acceptor molecule.

Figure 3

Uncoupling of Gα_i/o proteins from their cognate G protein-coupled receptor (GPCR). Activation of GPCRs leads to dissociation of heterotrimeric G protein complex into Gα_i/o and βγ subunit. The exchange of GTP from GDP results in activation of the inhibitory G protein (Gα_i/o), thereby inhibiting adenylyl cyclase (AC) activity. When the A-protomer of PTX penetrates into the host cells, the Gα_i/o is ADP-ribosylated at cysteine residue resulting in inactivation of Gα_i/o. The inhibitory effect of Gα_i/o on AC activity results in the elevation of intracellular cAMP levels, leading to activation of the cAMP-mediated signaling pathway. This enhanced pathway by PTX is recognized as the G_i/o protein-dependent pathway.

Figure 4

G_i/o protein-dependent and -independent effects of PTX. Following the binding of PTX to host cells, the A-protomer penetrates through the cell membrane. A-protomer is dissociated from B-oligomer and released into the cytoplasm. A-protomer then catalyzes the ADP-ribosylation of Gα_i/o that leads to uncoupling of Gα_i/o from its cognate inhibitory GPCRs. The inhibitory effect of Gα_i/o on AC activity is then halted, resulting in accumulation of cAMP. This action of PTX results in disruption of cellular function through cAMP-mediated signaling pathway (G_i/o protein-dependent effects). In a separate pathway, the B-oligomer binds to and interacts with several targeted proteins on the plasma membrane, leading to the induction of the biological responses that are independent of ADP-ribosylation of Gα_i/o protein (G_i/o protein-independent effects).