MOLECULAR CHARACTERIZATION OF A TYPE-I SERINE THREONINE KINASE RECEPTOR FOR TGF-BETA AND ACTIVIN IN THE RAT PITUITARY-TUMOR CELL-LINE GH3

GH3 pituitary tumor cells have surface receptors for transforming grow th factors-beta (TGF-beta s) and activins/inhibins. GH3 cell mRNA was screened by a novel reverse transcriptase-polymerase chain reaction te chnique with primers for receptor serine-threonine kinases. We isolate d rat homologs of previously identified clones for type I (ALK-2 and A LK-5) and type II (Ac-tRII, TGF-beta RII) activin and TGF-beta recepto rs, together with a novel clone, whose full-length version was isolate d from a GH3 cell cDNA library. Named B1, it encodes a 505-amino-acid protein belonging to the family of type I receptor serine/threonine ki nases. The kinase domain of B1 exhibits 90% identity to that of the TG F-beta type I receptor. B1 mRNA is expressed not only in pituitary cel ls but also in all other cells and tissues examined. B1 protein can be expressed on the cell surface, but cannot bind ligand unless a type I I receptor is also present. When coexpressed with the type II receptor s specific for TGF-beta or activin, B1 can be efficiently cross-linked to either ligand, suggesting that it can form heteromeric complexes w ith both type II receptor subunits. (C) 1995 Academic Press, Inc.