FIGURE 2.

Scheme and the spectra of the methyl-TCS experiments. A, schematic representation of methyl-TCS experiments. The saturation of the nonlabeled CXCR4 caused by the irradiation is transferred to the free state of the methyl-protonated SDF-1α mutant. B, schematic representation of the negative control experiments for methyl-TCS. The addition of equal amounts of deuterated wild-type SDF-1α to CXCR4 inhibits the specific interaction between CXCR4 and the methyl-protonated SDF-1α mutant. C and D, lower panel shows the two-dimensional ¹H-¹³C HSQC spectra of 100 μm [[U-²H[Ile^δ1-, Leu-, Val-¹³C¹H₃)]SDF-1α R8A/R12A with 10 μm nonlabeled CXCR4. D, 10 μm [U-²H]SDF-1α was further added (negative control experiments). Only the regions of the leucine methyl signals are shown (the full-size spectrum and assignments of each resonances are shown in supplemental Fig. 2). In the upper panels, the black lines show the ¹H one-dimensional slices through the Leu^55δ1 and Leu^62δ2 signals (the corresponding ¹³C frequencies are indicated with cyan lines in the two-dimensional spectra). The red lines show the same ¹H one-dimensional slices with selective irradiations of CXCR4.