New look at a tuberculosis protein machine

Understanding the molecular machines inside Mycobacterium tuberculosis is essential for basic science, but some assemblies are fragile and hard to capture. Markus A. Seeger led a study that focused on one such assembly, the MmpS4-MmpL4 complex, which plays a role in the bacterial cell envelope. Rather than relying on a single protein subunit, the researchers worked with a hexameric form made of interacting parts that are normally labile and difficult to study by traditional structural approaches. To overcome this challenge they used computational predictions to guide experimental stabilization, allowing them to obtain a clearer view of how the parts fit together. This allowed the team to move beyond structures of isolated monomers and see the full multiprotein complex in a state that is closer to how it might exist in the bacterium. The result is a new structural picture of the MmpS4-MmpL4 assembly that reveals features not visible in earlier monomer-focused studies.

The core technical advance in this work was to combine AlphaFold predictions with targeted chemistry to hold the complex together long enough for imaging. The team designed rational disulfide cross-links based on AlphaFold predictions to stabilize predicted protein interfaces, then used single particle cryo-EM to determine the structure of the hexameric MmpS4-MmpL4 complex. In the cryo-EM density they observed the coiled-coil domain protruding into the periplasmic space at roughly a 60° angle relative to the symmetry axis of the MmpL4 trimer. Comparing the hexameric assembly to previously observed monomeric MmpL4 revealed striking differences: the hexamer shows a large cavity in the periplasmic domain and rearrangements of conserved proton coupling residues in the transmembrane domain. These structural differences highlight how assembly state can alter protein shape and key functional elements, and they validate an experimental workflow for stabilizing labile multiprotein complexes for structural study.

The study provides two linked contributions: a detailed structural description of the MmpS4-MmpL4 hexamer from Mycobacterium tuberculosis and a general approach for studying fragile protein assemblies. The AlphaFold-informed disulfide cross-linking strategy is an example of how computational models can guide experimental design to trap protein interfaces that would otherwise fall apart. By revealing the coiled-coil orientation, the periplasmic cavity, and the shifted proton coupling residues in the transmembrane domain, the new structure shows that functional features depend on the assembled state. For researchers focused on bacterial membrane machines, this approach offers a path to obtain single particle cryo-EM structures of other labile multiprotein complexes. The workflow could be applied to similar systems where transient or weak interactions have so far prevented high-resolution structural analysis.