Abstract
Major intrinsic protein (MIP) superfamily contains water-transporting AQP1 and glycerol-specific GlpF belonging to two major phylogenetic groups, namely aquaporins (AQPs) and aquaglyceroporins (AQGPs). MIP channels have six transmembrane helices (TM1 to TM6) and two half-helices (LB and LE). LE region contributes two residues to the aromatic/arginine (Ar/R) selectivity filter (SF) within the MIP channel. Bioinformatics analyses have shown that all AQGPs have an intra-helical salt-bridge (IHSB) in LE half-helix and all AQGPs and majority of AQPs have helix destabilizing Gly and/or Pro in the same region. In this paper, we mutated in silico the acidic and basic residues in GlpF to Ser and introduced salt-bridge interaction in AQP1 LE half-helix by substituting Ser residues at the equivalent positions with acidic and basic residues. We investigated the influence of IHSB in LE half-helix on the transport properties of GlpF and AQP1 mutant channels using molecular dynamics simulations. With IHSB abolished in LE half-helix, the GlpF mutant exhibited a significantly reduced water transport. In contrast, the introduction of IHSB in the two AQP1 mutants has increased water transport. Absence of salt-bridge in LE half-helix alters the SF geometry and results in a higher energy barrier for the solutes in the Ar/R selectivity filter. Presence/absence of IHSB in LE half-helix influences the channel transport properties and it is evident especially for the AQGPs. By modulating its helical flexibility, LE half-helix can perhaps play a regulatory role in transport either on its own or in conjunction with other extracellular regions.
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Acknowledgements
We gratefully acknowledge Prof. Bert de Groot for sharing the Berger lipid parameters compatible with the OPLS-AA force-field. We thank the High Performance Computing facility of IIT-Kanpur supported by DST and MHRD, Government of India. RS is Pradeep Sindhu Chair Professor. RKV thanks Council of Scientific and Industrial Research (CSIR) for a Senior Research Fellowship. We thank all our lab members for suggestions and useful discussions.
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Alok Jain and Ravi Kumar Verma have contributed equally to this work.
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Jain, A., Verma, R.K. & Sankararamakrishnan, R. Presence of Intra-helical Salt-Bridge in Loop E Half-Helix Can Influence the Transport Properties of AQP1 and GlpF Channels: Molecular Dynamics Simulations of In Silico Mutants. J Membrane Biol 252, 17–29 (2019). https://doi.org/10.1007/s00232-018-0054-7
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DOI: https://doi.org/10.1007/s00232-018-0054-7