B61 IS A LIGAND FOR THE ECK RECEPTOR PROTEIN-TYROSINE KINASE

A PROTEIN ligand for the ECK1 receptor protein-tyrosine kinase has bee n isolated by using the extracellular domain (ECK-X) of the receptor a s an affinity reagent. Initially, concentrated cell culture supernatan ts were screened for receptor binding activity using immobilized ECK-X in a surface plasmon resonance detection system2,3. Subsequently, sup ernatants from selected cell lines were fractionated directly by recep tor affinity chromatography, resulting in the single-step purification of B61, a protein previously identified as the product of an early re sponse gene induced by tumour necrosis factor-alpha4. We report here t hat recombinant B61 induces autophosphorylation of ECK in intact cells , consistent with B61 being an authentic ligand for ECK. ECK is a memb er of a large orphan receptor protein-tyrosine kinase family headed by EPH5, and we suggest that ligands for other members of this family wi ll be related to B61, and can be isolated in the same way.