Bats are known for their robust immune systems. They have evolved enhanced tolerance towards viruses that can cause severe diseases in humans. Studying molecular adaptation in bats permits a better understanding of human antiviral immunity and the discovery of potential therapeutic targets against viral illnesses. In this project, we study Pteropus alecto (black fruit bat) MyD88, a central adaptor protein regulating Toll-like receptor (TLR) signalling during infection and immunity. MyD88 consists of an N-terminal death domain (DD) and a C-terminal Toll/Interleukin-1 receptor (TIR) domain. Upon activation, MyD88 is recruited to the cytoplasmic TIR domains of oligomerised TLRs via TIR:TIR interactions. Subsequently, MyD88 recruits the kinases IRAK4 and IRAK2 via DD:DD interactions, leading to the formation of the Myddosome complex, which further activates immune signalling.
To investigate the role of MyD88 in bat immunity, we sought to determine the structures of bat MyD88DD and MyD88TIR. Using cryo-EM, we found that bat MyD88DD forms an open-ended filamentous complex with a diameter of 70 Å and helical symmetry of 6.2 Å rise and 98.2° twist, similar to previously published human MyD88DD and Myddosome complexes. Comparison of human and bat MyD88DD complexes indicates that bat MyD88DD subunits form more stable interfaces. Our work on bat MyD88TIR suggests that bat MyD88TIR is more prone to complex formation (forming microcrystals) compared to human MyD88TIR. In the future, microcrystal electron diffraction (MicroED) will be used for bat MyD88TIR structure determination and site-directed mutagenesis will be performed to validate the function of bat MyD88 signalling complexes in cells.