Anales RANF

P.04 STRUCTURAL MODELS FOR DICTOSTELIUM DISCOIDEUM P2X RECEPTORS A. Alameer 1 , A. Stavros 2 , R. Schmid 2 1 . Kuwait University, Kuwait City, Kuwait; 2 . University of Leicester, Leicester, United Kingdom Our understanding of P2X receptors at a structural level has been hugely advanced by series of X-ray structures of P2X receptors in closed, open, and desensitized states (for review, see Pasqualetto et al., Front. Pharmacol., 9 , 58, (2018)). These structures can serve as templates for the generation of homology models for P2X receptors where structures have not been solved yet. Feasibility and accuracy of homology modelling is limited by the level of sequence similarity, typically sequence identity levels ~ < 35% are considered problematic. This threshold is not reached for P2X receptors from species very distant to mammals. The pairwise sequence identity to the probably best characterized protist P2X receptors from Dictostelium discoideum is ~ 22%. Nevertheless the low, but significant sequence similarity implies common ancestry, and indeed it has been shown that Dictostelium discoideum P2X receptors show characteristic features (e.g. activation by ATP, formation of stable trimers) and the overall shape with tri-fold symmetry has been demonstrated via electron microscopy. To gain further structural insight, we have used an integrated bioinformatics approach. For improving the quality of pairwise sequence alignments for homology derived structural constraints we employed profile Hidden Markov Models. Despite this, some regions, for instance the P2X receptor head region, remain unalignable and we applied ab initio modelling for “filling the gaps”. A series of alternative models are discussed in the context of the available functional data from literature.