C CA S51 CB S51 Hn Q52 CA S5/S43 CB S5/S43 Hn E44 13 C (ppm) 1 H (ppm) 13 C (ppm) 1 H (ppm) B Figure S1 A ERM AA# A.U SSP score Helix Strand 15 N ERM MED25 (1:0.1)

Figure S2 ERM AA#  1 H  15 N (ppm) A i3Q54 15 N ERM MED25 (1:0.2) i3Q54 15 N ERM MED25 (1:2) 1H1H 1H1H NH  W57 1H1H 15 N B

% Bound Figure S3 B  Bound Free Delta  k ex (~ K d ) A Delta  k ex (~ K d ) Bound Free

Figure S3 ERM/MED ERM/MED Gly524 Gly532 Gly436 Gly524 Gly532 Gly436 CD 1H1H 15 N 1H1H Gly532 Gly436 E F 1H1H 15 N 1H1H ERM-bound MED25 Free MED25 ERM-bound MED25

K d (µM) Relative CSD (%) n=10 r 2 =-0.97 K d (µM) Relative CSD (%) S N 1H1H A B Figure S4 15 N MED25 + ERM ERM F47L + ERM W57L + ERM F47L/W57L

Figure S5 Intensity ratio Bound/free I/I0 11 22 33 44 55 66 77 11 22 33 ERM F47L – MED25 WT 11 44 77 66 22 55 33 33 11 180° AB CD

Figure S6 Intensity ratio Bound/free I/I0 11 22 33 44 55 66 77 11 22 33 ERM W57L – MED25 WT 44 77 66 55 11 22 33 33 11 180° AB CD

1 H (ppm) 15 N (ppm) Cter-NO° B 1 H (ppm) 15 N (ppm) Nter-NO° A Figure S7

ERM F47L/W57L - MED25 Q451E 22 55 33 33 11 11 44 77 66 A B 180° CD Figure S8 Intensity ratio Bound/free MED25 11 22 33 44 55 66 77 11 22 33 Q451E MED25 AA#

Figure S9 Q451 H499 W57 BA