A new ephrin-A1 isoform (ephrin-A1b) with altered receptor binding properties abrogates the cleavage of ephrin-A1a

Ephrins are ligands for the Eph receptor tyrosine kinases, which play important roles in patterning nervous and vascular systems. Ephrin-Al is a glycosylphosphatidylinositol-anchored ligand that binds to the EphA receptor tyrosine kinases. In the present study, we have identified a new ephrin-Al isoform, denoted ephrin-Alb (ephrin-Al isoform b). Compared with the originally described ephrin-Al sequence. ephrin-Ala [Holzman, Marks and Dixit (1990) Moll. Cell. Biol. 10. 5830-5838], ephrin-Alb lacks a segment of 22 amino acids (residues 131-152). At the transcript level, exon 3 is spliced Out in the transcript encoding ephrin-Alb. Transfection of HEK-293T cells (human embryonic kidney 293 cells) with an ephrin-Allo-expressing plasmid resulted in a significant expression of the protein on the cell surface. However. soluble EphA2 receptor (EphA2-Fc) bound weakly to ephrin-Alb-expressing transfectants, but bound strongly to ephrin-Ala-expressing transfectants. Ephrins have been shown to undergo regulated cleavage after interaction with their receptors. This process is inhibited by co-expression of ephrin-Ala and ephrin-Alb, indicating that ephrin-Alb influences the cleavage process. Taken together, these findings indicate that this newly described isoform may regulate the function of its ephrin-Ala counterpart.