Expression and maturation of the cellular sea receptor, a member of the hepatocyte growth factor (HGF) receptor family of protein tyrosine kinases.

The c-sea proto-oncogene is a member of the Met/hepatocyte growth factor/scatter factor family of receptor protein tyrosine kinases. A distinguishing feature of this family, whose other member is the Ron/Stk receptor, is a novel heterodimeric structure. We have previously described cDNA clones encoding the avian Sea receptor. In this report we show that a full length c-sea cDNA directed the synthesis of a single 155 kDa polypeptide chain in vitro, while in vivo two polypeptides of 160 and 180 kDa were observed. We analysed the structure of the Sea receptor using a soluble chimeric protein consisting of the Sea extracellular domain linked to the hinge and constant regions of human IgG gamma 1. These studies indicated that the receptor undergoes proteolytic processing in the extracellular domain yielding an approximate 35 kDa alpha and a 160 kDa beta chain, and thus the Sea receptor appears to display a structure similar to that of the Met and Ron proteins. An examination of embryonic avian tissues using Sea extracellular domain-specific monoclonal antibodies revealed low levels of Sea receptor in a variety of tissues including kidney, intestine, liver, stomach, white blood cells and allantochorion. Elevated levels of expression were observed upon transformation of chicken embyro cells by the Src oncoprotein.