ONCOGENIC ACTIVATION OF V-KIT INVOLVES DELETION OF A PUTATIVE TYROSINE-SUBSTRATE INTERACTION SITE

The transforming gene of the Hardy-Zuckerman-4 strain of feline sarcom a virus, v-kit, arose by transduction of the cellular c-kit gene, whic h encodes the receptor tyrosine kinase (RTK) p145(c-kit). To gain insi ght into the molecular basis of the v-kit transforming potential, we c haracterized the feline c-kit by cDNA cloning. Comparison of the felin e v-kit and c-kit sequences revealed, in addition to deletions of the extracellular and transmembrane domains, three additional mutations in the v-kit oncogene product: deletion of tyrosine-569 and valine-570, the exchange of aspartate at position 761 to glycine, and replacement of the C-terminal 50 amino acids by five unrelated residues, Examinati ons of individual v-kit mutations in the context of chimeric receptors yielded inhibitory effects for some mutants on both autophosphorylati on and substrate phosphorylation functions. In contrast, deletion of t yrosine-569 and valine-570 significantly enhanced transforming and mit ogenic activities of p145(c-kit), while the other mutations had no sig nificant effects. Conservation in subclass III RTKs and the identifica tion of the corresponding residue in beta PDGF-R, Y579, as a binding s ite for src family tyrosine kinases suggests an important role for Y56 8 in kit signal regulation and the definition of its oncogenic potenti al, Repositioning of Y571 by an in-frame two codon deletion may be the crucial alteration resulting in enhancement of v-kit oncogenic activi ty.