Determination of the water network surrounding the type I pilus from <i>Escherichia coli</i> by cryo‐electron microscopy

Type 1 pili are protein filamentous surface structures of Gram-negative bacteria that mediate adhesion to host and play a crucial role in infection. Here, we report the cryogenic electron microscopy structure of the type 1 pilus from uropathogenic E. coli K-12 comprising 15 subunits of the major protein pilin FimA. The final local resolution of electron microscopy reconstruction was estimated to reach 1.85 Å, which is higher than that of the previously published structure. This improvement in the resolution enabled us to refine side-chain conformations to reliably determine the distances between the side-chain residues participating in the intersubunit interactions and determine a network of water molecules surrounding the pilus rod. The analysis revealed that water contributes to intersubunit stabilization both through discrete bridging interactions and through extended hydrogen-bonded clusters, thereby supporting both the rigidity and flexibility of the filament. Comparison with a homologous high-resolution pilus model from enterotoxigenic E. coli showed that the vast majority of 'conserved' water molecules, that is, those that are present at equivalent positions in different subunits of our model occupy also equivalent positions across the two structures, underscoring their functional relevance. At the same time, sequence-specific differences in hydration patterns were observed. These findings highlight the structural and functional importance of water in pilus architecture and provide a more detailed molecular framework for understanding bacterial adhesion.