Figure 3.

(A) Fraction of receptor in the activated state as the concentration of an inhibitory ligand increases. The ligand-binding parameters used in this homogenous two-state model are those estimated for the binding of an inhibitory attractant (α-methyl aspartate) to the aspartate receptor (K_D1 = 4 μM, K_D2 = 150 μM; see the text), while the K₁₂ value was varied as indicated. The fraction of receptor in the activated state (F₂) was calculated using the homogeneous two-state model as described by equation (4). (B) The experimentally determined effects of increasing concentration of ligand on the kinase activities of signaling complexes containing the QEEE, QEQE (wild-type), QQEQ, and QQQQ receptor modification states, determined using the in vitro receptor-coupled kinase assay under standard conditions (see the text).¹³ The measured kinase activities are directly proportional to the fractional occupancy of receptor in the activated state (F₂). For each modified receptor, the CheA kinase activity was measured at different concentrations of α-methyl aspartate. The resulting CheA kinase activities versus [attractant] data were then fit to a multisite Hill model (equation (11)). Each point is the average of at least three independent measurements. All indicated kinase activities are relative to that of the wild-type receptor (QEQE) in the absence of attractant, which is assigned a kinase activity of unity.