Super-Resolution Fluorescence Microscopy Reveals Clustering Behaviour of Chlamydia pneumoniae’s Major Outer Membrane Protein

SIMPLE SUMMARY: Chlamydia is an infamous sexually transmitted bacterium that also has a less well-known role in human respiratory infections, which has evolved a unique cell structure to enable its survival within the body. Covering the surface of this infectious cell is a strong mesh-like network made up of many different proteins which protects the cell against damage. This research focussed on the most abundant protein within this mesh, the Major Outer Membrane Protein (MOMP), and introduced a series of mutations designed to prevent the mesh from forming completely. The effect of the mutations was visualised by adding a bright fluorescent dye to each MOMP, which was then examined with a high-resolution fluorescence microscope capable of showing us each individual cell and the MOMPs at their surface. With statistical analysis, we observed that certain mutations disrupted the connections between MOMPs, giving us greater insight into how Chlamydia forms these interactions. Chlamydia is an extremely prevalent disease amongst the global population, and whilst treatable, there is currently no available vaccine. By researching Chlamydia’s biology and its method of evading our immune system, we can not only further our understanding of this complex bacterium, but also develop novel therapeutics for its treatment and prevention. ABSTRACT: Chlamydia pneumoniae is a Gram-negative bacterium responsible for a number of human respiratory diseases and linked to some chronic inflammatory diseases. The major outer membrane protein (MOMP) of Chlamydia is a conserved immunologically dominant protein located in the outer membrane, which, together with its surface exposure and abundance, has led to MOMP being the main focus for vaccine and antimicrobial studies in recent decades. MOMP has a major role in the chlamydial outer membrane complex through the formation of intermolecular disulphide bonds, although the exact interactions formed are currently unknown. Here, it is proposed that due to the large number of cysteines available for disulphide bonding, interactions occur between cysteine-rich pockets as opposed to individual residues. Such pockets were identified using a MOMP homology model with a supporting low-resolution (~4 Å) crystal structure. The localisation of MOMP in the E. coli membrane was assessed using direct stochastic optical reconstruction microscopy (dSTORM), which showed a decrease in membrane clustering with cysteine-rich regions containing two mutations. These results indicate that disulphide bond formation was not disrupted by single mutants located in the cysteine-dense regions and was instead compensated by neighbouring cysteines within the pocket in support of this cysteine-rich pocket hypothesis.