The adaptor protein CT10-Regulator of Kinase (CRK) and the closely related CRK-Like (CRKL) are adaptor proteins that play important roles in many signaling pathways regulating cell proliferation and cell motility. A notable example is their required role in Reelin signaling during development of the laminated structures of the vertebrate central nervous system, including the cerebral cortex, cerebellum, hippocampus, and retina. As adaptors, CRK/CRKL are important in coupling phosphotyrosine signaling to G protein activity to regulate both cell proliferation and changes in the actin cytoskeleton, thereby exerting control over cell motility, and migration. While many proteins that interact with CRK/CRKL have been identified, the diverse roles of these molecules suggest that more remain to be found. Herein is described a novel CRK/CRKL interacting protein, Src Homology 2 domain-containing protein D (SHD), which demonstrates a phosphorylation-dependent interaction with the CRK/CRKL SH2 domain in HEK 293 cells stimulated with hydrogen peroxide, which globally boosts tyrosine phosphorylation by inhibiting tyrosine phosphatases. Treatment with an inhibitor for Src family kinases (SFKs), Src-1, or an inhibitor of Abl/Arg kinases, STI571, reduces peroxide-induced binding of the CRKL-SH2 domain to SHD. We show that overexpression of Abl kinase, but not the SFK Fyn is sufficient to induce binding of the CRKL-SH2 to SHD and that this interaction requires at least one of the five tyrosines in YxxP motifs found in SHD. Using mass spectrometry, we found that Abl phosphorylates SHD on Y144, which is located in a YxxP motif. Mutation of this site to phenylalanine reduces, but does not prevent, Abl-induced binding of SHD to the CRKL-SH2 domain, suggesting that other YxxP sites also facilitate the interaction. A discussion of the cellular consequences of the interaction between SHD and CRK/CRKL is presented. To explore the biological role of SHD, we used the zebrafish to study shdb, a putative ortholog of human SHD. The expression of shdb was unknown and so we performed in situ hybridization and determined that shdb was expressed in the developing nervous system. To study the function of this gene, we used a morpholino to knock down expression of shdb which resulted in significantly reduced eye size. Possible roles of Shdb in eye development are discussed as is future research aimed to elucidate the cellular and developmental mechanisms by which Shdb functions in the developing eye.

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