Fig. 1. BMP-9 binds to ALK1 and endoglin in the absence of type-II receptors. COS-7 cells were transiently transfected with cDNAs for ALK1, ALK2, BMPR-II, ActRII-B or endoglin and affinity-labelled with [¹²⁵I]BMP-9. Crosslinked complexes were immunoprecipitated with specific antisera (marked with a circle) and subjected to SDS-PAGE and autoradiography. (A) [¹²⁵I]BMP-9 binds with high affinity to ALK1, but not to ALK2 in the absence of type-II receptors. BMP-9 binding to ALK2 is strongly enhanced when BMPR-II or ActR-IIB are co-expressed. (B) [¹²⁵I]BMP-9 binds to endoglin in the absence of type-I or type-II receptors.