An M protein coiled coil unfurls and exposes its hydrophobic core to capture LL-37.

ABSTRACT

Surface-associated, coiled-coil M proteins of Streptococcus pyogenes (Strep A) disable human immunity through interaction with select proteins. However, coiled coils lack features typical of protein-protein interaction sites, and it is therefore challenging to understand how M proteins achieve specific binding, for example, with the human antimicrobial peptide LL-37, leading to its neutralization. The crystal structure of a complex of LL-37 with M87 protein, an antigenic M protein variant from a strain that is an emerging threat, revealed a novel interaction mode. The M87 coiled coil unfurled and asymmetrically exposed its hydrophobic core to capture LL-37. A single LL-37 molecule was bound by M87 in the crystal, but in solution additional LL-37 molecules were recruited, consistent with a 'protein trap' neutralization mechanism. The interaction mode visualized crystallographically was verified to contribute significantly to LL-37 resistance in an M87 Strep A strain and was identified to be conserved in a number of other M protein types that are prevalent in human populations. Our results provide specific detail for therapeutic inhibition of LL-37 neutralization by M proteins.

We share our environment with many different bacteria. Some are beneficial for our health, like gut bacteria, but others can cause severe disease if they infect and spread within the body's tissues. For example, the bacterium Streptococcus pyogenes can cause conditions ranging from skin infections to a rapidly spreading deep-tissue infection, giving it the nickname "flesh-eating bacterium". To prevent infection, our bodies have developed defence mechanisms that target disease-causing bacteria. These include antimicrobial molecules, such as LL-37, which is a small protein produced on the skin. LL-37 kills bacteria by puncturing their cell membrane (the bacterial equivalent of our skin); in other words, it acts like a tiny chemical dart that 'pops' the bacterial cell. However, some bacteria, including S. pyogenes, can disarm these defences. S. pyogenes captures LL-37 on its surface with so called M proteins, which prevent LL-37 from reaching and destroying the underlying membrane. However, it was unknown how exactly the two proteins interact, especially since LL-37 is a simple molecule that lacks the structural features that allow most proteins to bind to each other. Kolesinski et al. set out to determine how the M protein can 'grab' LL-37. A technique called X-ray crystallography allowed them to visualise the molecules atom by atom and to examine the configuration of the M protein after it had captured LL-37. The M protein selected for these experiments (M87) came from a strain associated with particularly severe disease, considered to be an emerging health threat. The results showed that M87 uncurled itself, thereby exposing specific parts that normally remain hidden. This way, it could capture LL-37, like a hand opening to grab an object. Kolesinski et al. have revealed a key molecular mechanism that enables a disease-causing bacterium to invade our immune defences. Identifying which regions of M87 are involved in capturing LL-37 may help design more effective therapies to combat S. pyogenes infections.