Structure and properties of the selectivity filter of homo- and hetero- tetrameres of plant ionotropic glutamate receptors: a modelling approach

Plant glutamate receptor-like channels (GLRSs)control many plant-specific functions, including sperm chemotaxis in moss, pollen tube growth and biotic responses. Despite their physiological significance, GLRs molecular properties, such as ones behind ion permeation and ligand gating are not fully understood. We have evidence of a uniquely poor ionic selectivity, characterized by a strong anion coupled to weaker calcium permeability, and of possible roles for heteromerization in GLR regulation. In particular, AtGLR3.7 behaves as an obligatory heteromer that affects the ion permeation and gating of heteromers when it associates with other GLRs. The recent structural data obtained for two members of Arabidopsis thaliana GLRs(10.1016/j.str.2020.09.006, 10.1016/ j.molcel.2021.05.025)increased the robustness of modeling approaches for exploring GLR molecular function. Using protein modeling and molecular dynamics simulation, we inspected the region at the intersection of 4 GLR subunits that forms the ion permeation pathway and selectivity filter (SF)of AtGLR3.7 and AtGLR3.3. As a homomer, AtGLR3.7 possesses 8 positively charged residues in this region, pointing directly to the permeation pathway of the channel. We observed a high instability of AtGLR3.7 SF, which is stabilized by the application of an electrical field forcing anions to move across the permeation pathway. We conclude that positive residues present in the ion permeation pathway and SF of AtGLR3.7 pull anions toward the SF and stabilize it. This phenomenon was not observed in the AtGLR3.3 homomer (that does not display charged residue in SF vicinity), while it was pronounced in the AtGLR3.7/3.3 heteromer, suggesting that heterodimerization stabilizes the SF conformation by enhancing the movement of anions.We discuss the mechanism of GLR function regulation and diversification by heterodimerization.