Fig. 2.

Structural model of 5-HT4R with water molecules. A, three-dimensional model generated for 5-HT4R. Modified residues identified by MS footprinting analysis are represented as yellow sticks spatially distributed throughout this receptor. B, waters were placed into the 5-HT4R model by structural alignment of the main carbon skeleton to homologous class A GPCR crystal structures to analyze the general distribution of modified residues (see Fig. 5 for a detailed analysis of bound waters based on residue conservation); cyan balls from rhodopsin bound to 11-cis-retinal (PDB code 1F88); pink balls, β1-AR bound to dobutamine (PDB code 2Y00); dark blue balls, β1-AR bound to cyanopindolol (PDB code 2Y00); and golden brown balls, β2-AR bound to carazolol (PDB code 2RH1).