Question

Imagine that a mutation in the gene encoding the cholera toxin was made. This mutation affects the A-subunit, preventing it from interacting with any host protein. (a) Would the toxin be able to enter into the intestinal epithelial cell? (b) Would the toxin be able to cause diarrhea?

Short answer

(a) Yes, the toxin can enter the cell. (b) No, the toxin cannot cause diarrhea.

Step-by-step solution

Understanding Cholera Toxin Structure

Cholera toxin consists of two subunits, A and B. The B-subunit is responsible for binding to the surface of the intestinal epithelial cell and facilitating the entry of the A-subunit into the cell.

Function of A-Subunit

The A-subunit is the active part of the toxin, which interacts with host proteins to disrupt cellular mechanisms, leading to symptoms like diarrhea by causing an influx of water into the intestinal lumen.

Effect of Mutation on Entry

Since the mutation affects the A-subunit's interaction with host proteins but not the B-subunit, the toxin can still bind to and enter the intestinal epithelial cells.

Effect of Mutation on Symptoms

The mutated A-subunit can enter the cell but cannot interact with the necessary host proteins to disrupt cellular mechanisms. Therefore, it would not be able to cause diarrhea.

Key concepts

A-subunit function

The cholera toxin is composed of two main parts: the A-subunit and the B-subunit. Among these, the A-subunit is critical because it is the active component that disrupts normal cellular function. The way it does this is by interacting with certain host proteins inside the intestinal epithelial cells.
Once the cholera toxin binds to the cell surface through its B-subunit, the A-subunit gains entry into the cell. Inside, the A-subunit modifies a key protein called Gsα, which subsequently increases the levels of cyclic AMP (cAMP).
This increase in cAMP leads to the opening of chloride channels on the cell membrane, specifically the CFTR (cystic fibrosis transmembrane conductance regulator).
The influx of chloride ions into the intestinal lumen causes water to follow, creating the watery diarrhea characteristic of cholera.

mutation effect

Imagine if there is a mutation in the gene encoding the cholera toxin's A-subunit. This mutation prevents the A-subunit from interacting with any host proteins. Even if this happens, the B-subunit remains unaffected and can still bind to the surface of the intestinal epithelial cells.
As a result, the B-subunit can still facilitate the entry of the A-subunit into the cells. However, once inside, the mutated A-subunit will fail to modify the Gsα protein. This means that the cascade of events leading to increased cAMP and consequent diarrhea will not occur.
In summary, the mutated cholera toxin will be able to enter the intestinal epithelial cells but will not be able to exert its usual harmful effects and thus won't cause diarrhea.

intestinal epithelial cells

The intestinal epithelial cells are the primary target of the cholera toxin. These cells form the lining of the intestines and act as a barrier as well as a site for nutrient absorption and water balance.
These cells possess surface receptors that recognize and bind to the B-subunit of the cholera toxin. The binding process is very specific and allows the toxin to latch onto the cell surface. Once attached, the toxin is internalized, bringing the A-subunit into the cell.
Under normal circumstances, the A-subunit would interact with proteins within these cells to disrupt their function, leading to symptoms like severe dehydration and diarrhea, key characteristics of cholera.
Given how crucial these cells are for maintaining water and electrolyte balance, any disruption by choleral toxin greatly affects the body's ability to properly absorb and regulate fluids, causing the severe symptoms associated with cholera.