Characterization of the ret proto-oncogene products expressed in mouse L cells

The ret proto-oncogene (proto-ret) encodes a receptor type tyrosine kinase 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 fused to the SV40 promoter or cytomegalovirus (CMV) promoter into mouse L cells in which cadherins are not expressed. Three transfectants with high 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 fractionation experiments revealed that the 170 kDa protein but not the 150 kDa protein was detected predominantly in the plasma membrane fraction, indicating that the 170 kDa protein represents the mature glycosylated form of the proto-Ret protein present on the cell surface. Both 150 kDa and 170 kDa proto-Ret proteins showed tyrosine kinase activity in immunocomplex kinase assay. It is known that cadherins have Ca(2+)-dependent homophilic binding activity and are resistant to trypsinization in the presence of Ca2+. When L cells expressing the proto-Ret proteins were treated with trypsin in the presence of Ca2+, the 170 kDa protein was resistant to its digestion. On the other hand, it was completely digested in the presence of EGTA, suggesting the possibility that the proto-Ret protein interacts with Ca2+ like cadherins. However, the transfectants did not show clear adhesive properties in cell aggregation assays.