Drosophila myosin VI function in dorsal closure

Myosin VI is a minus-end directed actin-based motor protein. In vertebrate cells, myosin VI is of importance in endocytic and exocytic-membrane trafficking pathways, and in regulating Golgi morphology. Myosin VI is also implicated in genetic diseases and is transcriptionally regulated upon DNA damage in a p53-dependent manner and shown to be over-expressed in cancer types. In Drosophila, myosin VI encoded by the jaguar (jar) gene is implicated in a range of cellular processes including dorsal closure. Although understanding of its cellular function is developing, little is known about the mechanisms regulating myosin VI. In vitro studies on vertebrate myosin VI have demonstrated that it is phosphorylated within its motor domain by the serine/threonine kinase P AK and that alters its function in a similar manner to that which occurs in myosin I. More importantly,jar has a Pak phosphorylation site in the same location as the vertebrate myosin VI. Expression of either a dominant negative Jar or a dominant negative Pak causes if not exact, overlapping dorsal phenotypes in Drosophila. The aim of this study was to investigate a working model that Jar functions during dorsal closure are dependent on Pak-mediated phosphorylation. During my thesis research a published report concluded that jar is required but not essential for Drosophila development as 40% null jar mutants survived. On the contrary, I report here that jar is of importance for development as RNA i-mediated knockdown of Jar protein level is lethal and is found to cause abnormal dorsal closure. Further delineation of jar322 mutant allele showed varied phenotypes including wing defects that implicates jar in Notch signalling and in integrin function and signalling. Importantly, I found that Jar is directly regulated by JNK transcriptional activation pathway and down-regulated by the RhoGTPase pathway. Together this work emphasises the multi functional nature of myosin VI. Further the work suggests that the pro-survival function of vertebrate myosin VI is conserved in Drosophila.