Analysis of the function of Drop out, the single homologue of human MAST kinases, in Drosophila cellularisation
The Drop out (Dop) kinase is the single homologue of the MAST kinase family in Drosophila melanogaster. Despite MAST kinases having been implicated in several human diseases such as breast cancer, neurodegenerative diseases, inflammatory bowel disease and cystic fibrosis, the biological role of this kinase family is still poorly understood. The study of Dop function in Drosophila is therefore of interest to elucidate the function of MAST kinases. Previous studies revealed that mutations in dop affect cellularisation, a process during early Drosophila embryogenesis which occurs after 13 syncytial divisions. Cellularisation establishes cells by invagination of membrane from the cortex to surround about 6000 nuclei, thereby forming polarised cells in a blastoderm epithelium. dop mutant embryos display defects in polarity establishment, furrow formation and the focussing of furrow canal proteins at the leading edge of the invaginating membrane. Additionally, several studies suggest that Dop affects phosphorylation of Dynein and Dynein-dependent transport processes. In this study, analysis of complete loss-of-function mutants for dop reveals that Dop function is specifically required for cellularisation, but is dispensable for syncytial divisions. The first protein defects are visible just prior to cellularisation affecting Slam and Eps15 protein focussing into furrow structures. In contrast, the first morphological defect occurs slightly later at onset of cellularisation affecting furrow canal formation. Membrane invagination is highly reduced in complete loss-of-function dop mutants and Golgi as well as recycling endosome localisation are affected, suggesting a function of Dop in membrane recruitment via both Dynein and Kinesin microtubule motors. Furthermore, the localisation of the Dynein subunit Dynein light intermediate chain (Dlic) displays only minor changes in dop mutants and is not altered expressing phospho-mutant versions of the candidate Dop substrate Dlic Serine 401. However, preliminary data suggest that a phospho-mimic version of Dlic Serine 401 can reduce lethality of dop mutant embryos. This thesis suggests, that Dop affects cellularisation by regulating membrane recruitment to the plasma membrane, likely by affecting Dynein- and possibly Kinesin-dependent microtubule transport.
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