Cryo-EM structure of detergent-bound Vibrio cholerae MakA captures a candidate intermediate between the soluble form and mature assembly of an α-pore-forming toxin

Researchers have unveiled the structure of a key toxin from Vibrio cholerae, using cryo-electron microscopy. The toxin, called MakA, is part of a larger family of -pore-forming toxins that undergo significant shape changes when they interact with membranes. While scientists have previously mapped the structures of the separate components of MakA, MakB, and MakE, the way these pieces come together to form a functional toxin has been less clear.

In this new study, the scientists determined the structure of MakA when it's bound to a detergent, a process that mimics how it interacts with cell membranes. The MakA structure at 3.13 Angstrom resolution shows three MakA molecules arranged in a trimer, with each individual MakA subunit forming a helical shape that's different from the helical shape seen in the soluble form of the toxin.

This helical MakA subunit matches up with the building blocks found in similar toxins from other bacteria, suggesting that there might be a common way these toxins assemble.

Further analysis, including computer modeling and lab tests using liposomes, reveals that MakA is the main subunit that inserts into the cell membrane, MakE is somewhat involved in membrane association, and MakB remains mostly in a soluble state. The researchers propose that the MakA helical subunits serve as the initial unit for membrane insertion and as building blocks for assembling the toxin's pore structure.

The eventual assembly of the full toxin appears to start from this MakA helical arrangement, with MakA and MakE forming a trimer and then interacting with MakB to create the mature toxin.