Loss of Cysteine Rich domain was critical for evolution of heterodimerization in Toll proteins

Abstract Toll proteins play roles in immunity/development which have largely remained conserved. However, there are differences in Toll biology as mammalian TLRs recognise pathogen associated molecular patterns (PAMPs) but not their invertebrate homologues. The reason for the same is not known. One critical molecular difference is absence of Cysteine Rich Domain (CRD) in vertebrate Tolls and their presence in invertebrates. Interestingly, in Drosophila, all Toll proteins have CRD except Toll9. This provided us the appropriate model to investigate significance of loss of CRD in Toll evolution. CRDs nudge protein dimerization by forming disulphide bonds hence we asked if they did same in Drosophila Toll-proteins. We tested if, Toll-1(which forms homodimer) can heterodimerize with Toll-9. We found that wildtype Toll-1 didn’t interact with Toll9 however; when CRD of Toll1 was deleted/mutated it formed heterodimer with Toll9. This indicates that presence of CRD limits Toll proteins to form homodimer and thus its loss was a critical event which pushed Toll proteins towards heterodimerization. We further show that Drosophila Toll9 can directly bind dsRNA, a PAMP. Interestingly, dsRNA affinity for toll-9 monomer was twice as that for the dimer, which can be attributed to CRD loss. Thus, we show that loss of CRD was a major step in Toll evolution as it resulted in functional diversity and was a first step towards heterodimer formation. Therefore, we propose that CRD loss was under positive selection and also that heterodimerization of Toll-proteins is an evolved property. One line summary Loss of Cysteine Rich Domain in Drosophila Toll9 and recognition of dsRNA.