CHARACTERIZATION OF THE RET PROTOONCOGENE PRODUCTS EXPRESSED IN MOUSEL-CELLS

The ret proto-oncogene (proto-ret) encodes a receptor type tyrosine ki nase with a cadherin-related sequence in the extracellular domain. To investigate whether the proto-Ret protein functions as a cell adhesion molecule like cadherins, we transfected the human proto-ret gene fuse d to the SV40 promoter or cytomegalovirus (CMV) promoter into mouse L cells in which cadherins are not expressed. Three transfectants with h igh levels of expression of the proto-Ret proteins were obtained. The proto-Ret proteins were expressed as 150 kDa and 170 kDa glycoproteins in transfectants as observed in human neuroblastoma cells. Cell fract ionation experiments revealed that the 170 kDa protein but not the 150 kDa protein was detected predominantly in the plasma membrane fractio n, indicating that the 170 kDa protein represents the mature glycosyla ted form of the proto-Ret protein present on the cell surface. Both 15 0 kDa and 170 kDa proto-Ret proteins showed tyrosine kinase activity i n immunocomplex kinase assay. It is known that cadherins have Ca2+-dep endent homophilic binding activity and are resistant to trypsinization in the presence of Ca2+. When L cells expressing the proto-Ret protei ns were treated with trypsin in the presence of Ca2+, the 170 kDa prot ein was resistant to its digestion. On the other hand, it was complete ly digested in the presence of EGTA, suggesting the possibility that t he proto-Ret protein interacts with Ca2+ like cadherins. However, the transfectants did not show clear adhesive properties in cell aggregati on assays.