Fig. 4

BMP type I receptor discrimination by GDF-5. a The β1β2-loop of the BMP type I receptor BMPR-IA (Cα-trace in cyan) and BMPR-IB (gray) adopt different conformations. Two alternative conformations are observed for this loop in the complex GDF-5:BMPR-IB; in contrast, the β1β2-loop of BMPR-IA bound to GDF-5 R57A adopts a single locked conformation that is impossible if a large arginine side chain is at position 57 (as for example in GDF-5WT). b As in (a) but rotated around the y-axis by 90 ° showing the putative steric clash between the side chains of His43 (cyan) in the β1β2-loop of BMPR-IA and Arg57 of wild-type GDF-5 (green). c The β1β2-loop of BMPR-IA (cyan) bound to GDF-5 R57A and BMPR-IA (blue) bound to BMP-2 adopt identical backbone and side-chain conformations