ABL drives phosphorylation of Semaphorin 6 A at conserved tyrosines necessary for zebrafish retinal maintenance

Semaphorin 6A (SEMA6A) and Plexin A2 (PLXNA2) are interacting transmembrane proteins dually serving as both receptor and ligand. SEMA6A drives forward signaling through Plexin A2 while PLXNA2 drives reverse signaling through SEMA6A. We have shown in zebrafish that forward signaling is essential for proper retinal size and lamination, whereas reverse signaling is necessary for retinal maintenance. To further characterize reverse signaling, we performed mass spectrometry analysis of immunopurified murine SEMA6A co-expressed with ABL, a tyrosine kinase implicated in SEMA6A reverse signaling. ABL induced SEMA6A phosphorylation at three intracellular tyrosines, two of which are conserved across vertebrates. Expression of murine wild-type SEMA6A mRNA, but not a tyrosine-to-phenylalanine mutant at the three sites, rescued reverse signaling retinal defects observed in sema6a morphants.