How Does Pertussis Toxin Inhibit G Proteins?
Examine the molecular mechanism by which Pertussis Toxin chemically modifies and deactivates inhibitory G proteins, disrupting essential cell signaling pathways.
Pertussis Toxin (PTX) is a primary virulence factor from the bacterium Bordetella pertussis, the organism responsible for causing whooping cough. It is an A-B type exotoxin with a two-component structure. Beyond its role in infectious disease, the toxin has become a tool for cell biology research due to its highly specific effect on host cells, which allows for detailed investigation of cellular communication pathways.
G Protein-Coupled Signaling Pathways
G proteins are a family of proteins that act as molecular switches inside cells, transmitting signals from outside a cell to its interior. They are central to how cells respond to their environment, including hormones, neurotransmitters, and sensory stimuli. These proteins exist in an inactive state until a signal binds to a corresponding G protein-coupled receptor (GPCR) on the cell surface. This interaction activates the G protein, which then initiates a cascade of further signals within the cell.
A specific subgroup of these, known as inhibitory G proteins (Gi/o), serves a repressive function. When a receptor coupled to a Gi/o protein is activated, its alpha subunit detaches and interacts with an enzyme called adenylyl cyclase. This interaction inhibits the enzyme, preventing it from producing the signaling molecule cyclic AMP (cAMP). The result is a decrease in the intracellular concentration of cAMP, acting as an “off-switch” for specific cellular processes.
The Molecular Mechanism of Pertussis Toxin
The Pertussis Toxin molecule has an A-B structure common to many bacterial toxins, consisting of one “A” subunit and five “B” subunits. The B-oligomer, composed of subunits S2 through S5, is responsible for binding to receptors on a host cell’s surface. Once bound, the entire toxin is taken into the cell through endocytosis and transported to the endoplasmic reticulum.
Inside the cell, the A-protomer, also known as the S1 subunit, becomes enzymatically active. This active component carries out a chemical reaction known as ADP-ribosylation, targeting the alpha subunit of the inhibitory Gi/o proteins. The S1 subunit transfers an ADP-ribose group from nicotinamide adenine dinucleotide (NAD+) to a specific cysteine residue on the Gi/o alpha subunit.
This chemical modification blocks the Gi/o protein from interacting with its G protein-coupled receptor. This prevents the G protein from being activated, locking it in an inactive, GDP-bound state. As a result, the G protein can no longer perform its normal function of inhibiting adenylyl cyclase, even when the cell surface receptor is stimulated.
Cellular Disruption by Toxin Action
The permanent inactivation of the inhibitory G protein has significant consequences for the cell’s internal signaling. With the Gi/o protein unable to function, the “brake” on the adenylyl cyclase enzyme is removed. This loss of inhibition leads to the uncontrolled production of cyclic AMP (cAMP). Adenylyl cyclase continuously converts ATP into cAMP, causing the intracellular concentration of this signaling molecule to rise dramatically. This accumulation disrupts the cell’s signaling network and affects various downstream cellular processes.
Pathophysiological and Research Applications
The cellular disruption caused by the toxin translates into the symptoms of whooping cough. The elevated cAMP levels in respiratory tract cells are thought to contribute to symptoms like increased mucus secretion and the characteristic cough, although the direct link is still under investigation. In immune cells, the high cAMP inhibits functions like chemotaxis, preventing neutrophils and macrophages from migrating to the site of infection. This allows Bordetella pertussis to colonize the respiratory tract more easily.
Scientists have harnessed the toxin’s specific mechanism as a research instrument. Because PTX exclusively targets and inactivates Gi/o proteins, it is used in laboratories to determine if a particular cellular signaling pathway relies on this type of G protein. If treating cells with Pertussis Toxin blocks a specific biological response, it provides strong evidence that the pathway is initiated by a Gi/o-coupled receptor.