Table 2.

Effects of receptor modifications on the apparent CheA kinase affinity of the signaling complex

Receptor modification	$K_{\text{D}}^{\text{complex}}$ for CheA binding (μM monomer)a	$V_{\text{sat}}^{\text{apo}}$ (relative)b	$F_{2\text{sat}}^{\text{apo}}$ c
QEEE	0.40 ± 0.09	0.6 ± 0.1	0.075
QEQE	0.34 ± 0.04	1.7 ± 0.2	0.21
QQQQ	0.23 ± 0.03	4.1 ± 0.3	0.51
G278V/QQQQ	0.25 ± 0.02	8.0 ± 0.5	1.0

^a $K_{\text{D}}^{\text{complex}}$ is the apparent dissociation constant for CheA binding to the signaling complex determined by best-fit Hill analysis of Figure 6(A). For a fixed receptor concentration (2 μM), the concentrations of CheA kinase, CheW, and CheY were covaried at constant molar ratios (CheA:CheW:CheY was 1:4:20). All protein concentrations are given in monomeric units.

^b $V_{\text{sat}}^{\text{apo}}$ is the maximal kinase activity attained under saturating conditions in the absence of attractant, relative to the kinase activity of the QEQE receptor under standard conditions, which is set equal to unity (see $V_{\text{obs}}^{\text{apo}}$ for QEQE in Table 1). The $V_{\text{sat}}^{\text{apo}}$ parameter is determined by best-fit Hill analysis of Figure 6(A) as indicated for $K_{\text{D}}^{\text{complex}}$. No significant kinase activity was observed for EEEE.

^c $F_{2\text{sat}}^{\text{apo}}$ is the fractional population of the on-state in the absence of attractant at saturating levels of CheA (as well as CheW and CheY). This parameter is calculated as the ratio $F_{2\text{sat}}^{\text{apo}}=V_{\text{sat}}^{\text{apo}}/V_{2\text{sat}}$, where the kinase activity observed when the entire population is in the on-state (V_2sat) is operationally defined as the kinase activity of G278V/QQQQ under saturating conditions.