Identification of the human ltk gene product in placenta and hematopoietic cell lines

Two different monoclonal antibodies (MAbs) were raised against an extracellular domain and a C-terminal portion of the human ltk protein which is a receptor-type protein tyrosine kinase. Western blot analysis showed that these MAbs specifically immunoprecipitated a 100 kDa ltk protein which was transiently expressed in COS-1 cells transfected with a human ltk cDNA. By an in vitro immune complex kinase assay using these MAbs, a 100 kDa phosphoprotein was detected in human placenta and hematopoietic cell lines. These data indicate that the ltk gene product expressed in human placenta and hematopoietic cells shows tyrosine kinase activity. This is the first detection of native ltk protein naturally expressed in human cells.