(A) Sequence alignment of SCTR and GLP-1R, specifically TM5, ICL3 and
TM6 and highlighting homolog residues C347GLP−1R and
K317SCTR; generated by GPCRdb. cAMP dose-response of (B, C, top
panels) GLP-1, (B, C, bottom panels) GLP-1(9–36), Sec-FL (D, E, top
panels) or Sec(3–27) (D, E, bottom panels) stimulating (B, C, left
panels) GLP-1R-WT, (B, C, right panels) GLP-1R C347A, (D, E, left panels) SCTR
WT, (D, E, middle panels) SCTR K317A or (D, E, right panels) SCTR K317C treated
with DMSO (black), 12.5–25 μM of (B, D) BETP (purple)
or 2-sulfonyl pyrimidines A1 (dark green), A2 (light
green), A3 (light blue-green) or (C, E) 2-mercapto pyrimidines
B1 (light blue), B2 (dark blue); TR-FRET ratios
resulting from cAMP accumulation normalized to corresponding ligands; graphs
plotted using GraphPad Prism; experiments performed in duplicate in at least
three independent experiments; data points shown as mean ± SEM.
Statistical significance for EC50 shifts determined using GraphPad
Prism’s unpaired t-test corrected with Holm-Sidak method, α =
0.01; all results statistically significant (p < 0.01) if not indicated
otherwise (ns = not significant).