CHARACTERIZATION OF CHICKEN KIT TYROSINE KINASE RECEPTOR IN COS CELL TRANSFECTANTS AND IN CHICKEN BRAIN

The Kit tyrosine kinase (Kit) encoded by the c-kit proto-oncogene is a receptor for stem cell factor (SCF). Kit proteins of mice and humans are expressed in various kinds of hematopoietic progenitor cells and a re essential for the growth of these cells. Wild-type chKit (chKit+) a nd a mutant chKit (chKit42) that contained an amino acid change from A sp(777) to Asn corresponding to that in Kit of the W-42 mutant mice we re produced in Cos-l cells transfected with expression plasmids contai ning the chicken c-kit cDNA, and characterized using two kinds of anti -chKit antisera. The W-42 mutant Kit has previously been shown to be d efective for kinase activity. The chKit+ of 145 kilodalton (kDa) and 1 30 kDa with varying degrees of N-linked glycosylation were detected. W estern blot analysis using an anti-phosphotyrosine monoclonal antibody showed that autophosphorylation of chKit42 was greatly enhanced upon chicken SCF induction. The chKit+ did not respond to mouse SCF. The ki nase activity of chKit42 was abolished by the amino acid substitution, indicating the Asp(777) residue was essential for the activity. In ad dition, 145 kDa chKit conjugated with sialic acid residue(s) was detec ted in chicken brain by immunoprecipitation using the antisera. An in vitro kinase assay showed the kinase activity of this protein. These s tructural and functional similarities of chKit to mammalian Kit protei ns shown in this study implicate a possible role of chKit in chicken h ematopoietic system.