Mechanistic insights into glycosylation-driven structural rearrangements in human aquaporin 1

The asparagine-linked glycosylation site of the extended extracellular loop connecting transmembrane helices 1 and 2 has been identified in human aquaporin 1 (AQP1), though its functional significance remains unclear. Here, we investigate AQP1 glycosylated at Asn42 and Asn205, using molecular dynamics simulations. In glycosylated AQP1, fluctuation of the protein backbone groups surrounding the linker Asn42 in the extended extracellular loop 1-2 is significantly suppressed compared to non-glycosylated AQP1. The heavily hydrated glycan at Asn42 stretches the extracellular loop, which in turn induces conformational changes that propagate into the transmembrane region. Notably, glycosylation reduces water permeability by narrowing the pore near the extracellular entrance, thereby lowering water occupancy and permeation frequency. These structural rearrangements are accompanied by glycosylation-induced displacements of helices 1 and 5, which alter hydrogen-bond interactions involving residues such as His180 and Arg195, components of the aromatic/arginine selectivity filter. These findings reveal how extracellular glycosylation directly modulates AQP1 channel architecture and function, suggesting a general regulatory mechanism for aquaporin family members bearing glycosylation sites in extracellular loops.