Hidetoshi Yamashita

Bone morphogenetic proteins (BMPs) are multifunctional proteins, structurally related to transforming growth factor-beta (TGF-beta) and activin. TGF-beta and activin exert their effects by forming heteromeric complexes of type I and type II serine/threonine kinase receptors. We have previously identified a series of type I serine/threonine kinase receptors, termed activin receptor-like kinase (ALK)-1 to -6. ALK-5 is a TGF-beta type I receptor, whereas ALK-2 and ALK-4 are activin type I receptors. Here we investigated the binding of proteins in the BMP family to ALKs. In transfected COS cells, the binding of osteogenic protein (OP)-1 and BMP-4 to certain ALKs was observed in the absence of type II receptors, and their binding was increased after co-transfection of a BMP type II receptor from Caenorhabditis elegans, DAF-4. OP-1 bound to ALK-2 and ALK-6 efficiently, and to ALK-3 less efficiently, whereas BMP-4 bound to ALK-3 and ALK-6 efficiently. Similarly, OP-1 bound to ALK-2, ALK-3, and/or ALK-6 in various nontransfected cell lines, although the binding profiles were different between different cell types. BMP-4 bound to ALK-3 in MC3T3-E1 osteoblasts and human foreskin fibroblasts. These results suggest that ALK-3 and ALK-6 are type I receptors for OP-1 and BMP-4; in addition, ALK-2 is a type I receptor shared by activin and OP-1, but not by BMP-4.

Transforming growth factor-beta (TGF-beta) and activin exert their effects by binding to heteromeric complexes of type I and type II receptors. The type II receptors for TGF-beta and activin are transmembrane serine-threonine kinases; a series of related receptors, denoted activin receptor-like kinase (ALK) 1 to 5, have recently been identified, and ALK-6 is described here. ALK-5 has been shown to be a functional TGF-beta type I receptor. A systematic analysis revealed that most ALKs formed heteromeric complexes with the type II receptors for TGF-beta and activin after overexpression in COS cells; however, among the six ALKs, only ALK-5 was a functional TGF-beta type I receptor for activation of plasminogen activator inhibitor-1, and only ALK-2 and ALK-4 bound activin with high affinity.

Bone morphogenetic proteins (BMPs) are members of the transforming growth factor beta superfamily. Several members of this family have been shown to transduce their signals through binding to type I and type II serine-(threonine) kinase receptors. Here we report the cDNA cloning and characterization of a human type II receptor for BMPs (BMPR-II), which is distantly related to DAF-4, a BMP type II receptor from Caenorhabditis elegans. In transfected COS-1 cells, osteogenic protein (OP)-1/BMP-7, and less efficiently BMP-4, bound to BMPR-II. BMPR-II bound ligands only weakly alone, but the binding was facilitated by the presence of previously identified type I receptors for BMPs. Binding of OP-1/BMP-7 to BMPR-II was also observed in nontransfected cell lines. Moreover, a transcriptional activation signal was transduced by BMPR-II in the presence of type I receptors after stimulation by OP-1/BMP-7.

Proteins in the TGF-beta superfamily transduce their effects through binding to type I and type II serine/threonine kinase receptors. Osteogenic protein-1 (OP-1, also known as bone morphogenetic protein-7 or BMP-7), a member of the TGF-beta superfamily which belongs to the BMP subfamily, was found to bind activin receptor type I (ActR-I), and BMP receptors type IA (BMPR-IA) and type IB (BMPR-IB) in the presence of activin receptors type II (ActR-II) and type IIB (ActR-IIB). The binding affinity of OP-1 to ActR-II was two- to threefold lower than that of activin A. A transcriptional activation signal was transduced after binding of OP-1 to the complex of ActR-I and ActR-II, or that of BMPR-IB and ActR-II. These results indicate that ActR-II can act as a functional type II receptor for OP-1, as well as for activins. Some of the known biological effects of activin were observed for OP-1, including growth inhibition and erythroid differentiation induction. Compared to activin, OP-1 was shown to be a poor inducer of mesoderm in Xenopus embryos. Moreover, follistatin, an inhibitor of activins, was found to inhibit the effects of OP-1, if added at a 10-fold excess. However, certain effects of activin, like induction of follicle stimulating hormone secretion in rat pituitary cells were not observed for OP-1. OP-1 has overlapping binding specificities with activins, and shares certain but not all of the functional effects of activins. Thus, OP-1 may have broader effects in vivo than hitherto recognized.