Structural insights into the role of deleterious mutations at the
dimeric interface of TRAM protein
Abstract
Toll-like receptors (TLRs) are major players of the innate immune system
– recognizing pathogens and differentiating self/non-self components of
immunity. These proteins are present either on the plasma membrane or
endosome and recognise pathogens at their extracellular domains. They
are also characterised by a single transmembrane helix and an
intracellular TIR domain. Few TIRs directly invoke downstream
signalling, while others require other TIR domains of adaptors like TRAM
and TRIF. On recognizing pathogenic lipopolysaccharides (LPS), TLR4
dimerises and interacts with the intracellular TRAM dimer through the
TIR domain to further recruit TRIF molecules. We have performed an
in-depth study of the effect of two mutations, P116H and C117H, at the
dimeric region of the adaptor TRAM, which are known to abrogate
downstream signalling. We modelled the structure and performed molecular
dynamics studies to infer the structural changes occurring across the
trajectory due to the point mutations in order to decipher the
structural basis of this dramatic effect. We observed that these
mutations led to increased RoG (Radius of Gyration) of the complex and
resulted in several changes to the interaction energy values when
compared against the wild type and few positive control mutants. We
identified highly interacting residues as hubs and few such hubs that
were lost in the mutant dimers. Further, changes in the protein residue
path, hampering the information flow between the crucial AEDD and TS
sites, happen for the mutants. Overall, we show that such residue
changes can have subtle but long-distance effects, impacting the
signaling path allosterically.