Shark tyrosine kinase-interacting proteins in Drosophila embryonic development
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Embryonic dorsal closure (DC) in Drosophila is a series of morphogenetic movements involving the bilateral dorsal movement of the epidermis (cell stretching) and dorsal suturing of the leading edge (LE) cells to enclose the viscera. The Syk family tyrosine kinase, Shark, plays a crucial role in this Jun amino-terminal kinase (JNK)-dependent process, where it acts upstream of JNK in LE cells. Using a yeast two-hybrid screen three Shark-interacting proteins were identified: The unique Drosophila homolog of the downstream of kinase (Dok) family Ddok, Draper and Bip2 (Bric-a-brac interacting protein-2). The interaction of Shark with either Ddok or Draper was tyrosine phosphorylation-dependent. Bip2 interaction with Shark was independent of tyrosine phosphorylation. The embryonic expression patters of Ddok and draper resemble the expression pattern of shark.;Ddok loss-of-function mutant (DdokPG155 ) germ-line clones possessed DC defects, including the loss of Jun amino-terminal kinases-dependent expression of dpp mRNA in LE cells, decreased epidermal F-actin staining and LE actin cable formation. Epistatic analysis indicated that Ddok functions upstream of shark to activate JNK signaling during DC. Consistent with these observations, Ddok mutant embryos exhibited decreased levels of tyrosine phosphorylated Shark at the cell periphery of LE and epidermal cells. As there are six mammalian Dok family members that exhibit some functional redundancy, analysis of the regulation of DC by Ddok is expected to provide novel insights into the function of the Dok adapter proteins.;A Loss-of-function mutant of draper and draper RNAi embryos do not show a DC defect. However draper mutant larvae exhibited delay cell corpse removal after neuronal injury. A similar phenotype was also observed in shark mutants. However, overexpression of Draper in embryos with different drivers results in phenotypes such as split-thorax, loss of crossveins in wings and thick wings reflecting increased cell proliferation or decreased apoptosis. As draper homozygote females are sterile it is not possible to obtain draper germline clone embryos, it is possible that Draper functions as a receptor regulating the JNK-dependent embryonic dorsal closure.