In this video, we're going to discuss our first major group of exotoxins, which are the AB toxins. The AB toxins, once again, are exotoxins, and they actually consist of 2 parts as their name implies with A and B. The two parts of the AB toxin are the A subunit and the B subunit. Now the A in A subunit stands for the A in active, and so the A subunit is also known as the active subunit. It turns out that this A subunit is usually going to be an enzyme and it is going to be responsible for all of the toxic damaging activity of the AB toxin. Of this entire AB toxin, it's really the A or active subunit that is going to be responsible for the toxic damaging activity. Now the B subunit is going to also be known as the binding subunit. The B stands for the B in binding. And the B or binding subunit is not necessarily toxic. As its name implies, it's going to dictate the very specific type of cell that the AB toxin will bind to. The B subunit or the binding subunit is going to dictate the binding of the AB toxin. And because once again the B or binding subunit is not necessarily toxic, it's the A subunit that is toxic. What this means is that the B subunit can be isolated and separated from the A subunit and then the B subunit can be recombined with medicines so that those medicines can be delivered to very specific cell types. This is an area that is being researched and explored even today.
The AB toxins can actually infect host cells in a 3 step process that's being shown below in this image. If we take a look at this image down below, notice over here on the far left hand side, we're showing you the AB toxin right here at this location. Once again, the AB toxin has 2 subunits. It has the A subunit, or the active subunit, which is the portion that's responsible for the toxic damaging activity. And then it has the B subunit or the binding subunit, which is not necessarily toxic, but is important for binding to specific cell types, binding to specific receptors on specific host cells. What you can see here is in the first step, the B subunit binds specifically to a receptor on the host cell surface, and that's exactly what we see over here. The B subunit is binding to the cell receptor on the host cell. Now in the second step of AB toxin process, both the A and the B subunits are going to enter the cell via endocytosis. Here in this image, you can see that both the A and the B subunits of the AB toxin are going to be internalized into the host cell. Recall that when endocytosis occurs, these AB toxins are going to be brought into the cell in an endocytic vesicle. That's exactly what we see over here on the right hand side is the endocytic vesicle that results from endocytosis. Both the A and the B subunits are inside of this endocytic vesicle. However, the A subunit, which is again the active subunit, is able to leave the endocytic vesicle and enter into the cytoplasm of the cell. In the cytoplasm of the cell, the A subunit is going to be active and it can target cellular functions and inhibit cellular functions such as inhibiting translation here. Translation is being blocked and this mRNA cannot be translated into the protein and that is because of the catalytic activity of this A subunit of the AB toxin. Blocking translation could potentially be toxic and cause damage to that cell. This concludes our brief lesson on AB toxins, and we'll be able to get some practice applying these concepts as we move forward. So I'll see you all in our next video.
