Ligand binding to the serine receptor of Escherichia coli has been studied using isothermal titration calorimetry. Bacterial inner membranes enriched in the serine receptor (Tsr) were titrated as sonicated membrane samples and after solubilization in octyl beta-D-glucopyranoside (OG) to determine the number of moles of ligand bound per mole of receptor (n), the binding constant (Ka), and the enthalpy of binding (delta H) of serine to the receptor. The n value for serine binding to OG-solubilized Tsr protein (n = 0.5) was consistent with one molecule of serine binding to a receptor dimer, but in sonicated inner membrane samples, the n value was smaller (n approximately equal to 0.25), indicating that not all of the binding sites were accessible to added serine. At 7 and 27 degrees C, the values for Ka and delta H were equivalent for the membrane and OG-solubilized samples and were found to be 4.7 x 10(4) M-1 and -15 kcal/mol, and 3.6 x 10(4) M-1 and -18 kcal/mol, respectively. The influence of covalent modification at the sites of methylation on the affinity of the receptor for serine was also investigated, and found to have only a modest effect. The property of half-site saturation is suggestive of models for transmembrane signaling where the receptor subunit interactions are modulated by ligand binding.