Analysis of the native murine bone morphogenetic protein serine threonine kinase type I receptor (ALK-3).

The bone morphogenetic proteins, members of the transforming growth factor-beta cytokine family, induce the osteoblast phenotype and promote osteogenesis in the bone marrow stroma. Simultaneously, these cytokines inhibit other mesodermal differentiation pathways, such as adipogenesis and myogenesis. The receptors for the bone morphogenetic proteins belong to a family of transmembrane serine/ threonine kinase TGF beta type I and type II receptor proteins. In man, these include the activin receptor like kinase-3 (ALK-3), a type I receptor protein. We have used a polyclonal antibody to examine the expression of the native murine ALK-3 protein in murine tissues and bone morphogenetic protein-responsive cell lines. On Western blot analyses, we found that the native 85 kDa native ALK-3 protein was expressed in a number of murine tissues; protein and mRNA levels did not necessarily correlate. Two bone morphogenetic protein-responsive cell lines, BMS2 bone marrow stromal cells and C2C12 myoblasts, expressed the ALK-3 protein constitutively. Cell differentiation was accompanied by modest changes in ALK-3 protein levels. Immunoprecipitation of the ALK-3 protein cross linked to [125I] BMP-4 revealed two major receptor complexes of approximately 90 kDa and 170 kDa in size. Biotin surface-labeling experiments revealed that the 85 kDa ALK-3 protein was constitutively associated with a novel 140 kDa surface glycoprotein. Deglycosylation reduced the protein's size to 116 kDa, comparable in size to that of the recently described BMP type II receptor. These findings support the current model that BMP interacts with a pre-existing complex consisting of a type I and type II receptor protein.