The Drosophila SH2-SH3 adapter protein Dock is expressed in embryonic axons and facilitates synapse formation by the RP3 motoneuron

The Dock SH2-SH3 domain adapter protein, a homolog of the mammalian Nck onc oprotein, is required for axon guidance and target recognition by photorece ptor axons in Drosophila larvae. Here we show that Dock is widely expressed in neurons and at muscle attachment sites in the embryo, and that this exp ression pattern has both maternal and zygotic components. In motoneurons, D ock is concentrated in growth cones. Loss of zygotic dock function causes a selective delay in synapse formation by the RP3 motoneuron at the cleft be tween muscles 7 and 6, These muscles often completely lack innervation in l ate stage 16 dock mutant embryos, RP3 does form a synapse later in developm ent, however, because muscles 7 and 6 are normally innervated in third-inst ar mutant larvae. The absence of zygotically expressed Dock also results in subtle defects in a longitudinal axon pathway in the embryonic central ner vous system. Concomitant loss of both maternally and zygotically derived Do ck dramatically enhances these central nervous system defects, but does not increase the delay in RP3 synaptogenesis. These results indicate that Dock facilitates synapse formation by the RP3 motoneuron and is also required f or guidance of some interneuronal axons The involvement of Dock in the conv ersion of the RP3 growth cone into a presynaptic terminal may reflect a rol e for Dock-mediated signaling in remodeling of the growth cone's cytoskelet on.