Book Summary:
Gastrulation in Drosophila involves a number of cell shape changes, the first of which is the formation of the ventral furrow, an invagination on the ventral surface of the embryo that internalizes the mesodermal precursors. In order to identify the mechanical mediators of this process, most of which have remained unidentified by conventional genetic methods, we have taken a comparative proteomic approach. Using difference gel electrophoresis (DIGE), we identified a total of 37 novel proteins that could potentially play a critical role in ventral furrow formation. These included a number of functionally different proteins ranging from proteases to cytoskeletal proteins. Three of these proteins were subunits of the 26S proteasome. Their role was confirmed using RNAi, wherein knocking down the levels of these proteins led to defects in ventral furrow formation. Proteome analysis of embryos blocked in ventral furrow formation by RNAi of the ventral-specific proteasome proteins revealed changes in the iron transport protein, Transferrin (Tsf1). Altering Tsf1 levels also blocked ventral furrow formation. Since Tsf1 is an extracellular protein, it is an unlikely target of the proteasome. We, therefore, examined the role of Iron Regulatory Proteins (IRPs). RNAi of IRP-1A, but not IRP-1B, caused ventral furrow defects. Proteome analysis showed that RNAi of IRP-1A caused changes in TSF1 levels. Surprisingly, this treatment also abrogated the ventral-specific proteasome changes. Thus, there appears to be a regulatory loop where the ventral-specific proteasome controls iron homeostasis and changes in iron homeostasis conversely regulate proteasome changes. The Toll signaling pathway feeds into the regulatory loop, which in turn controls specific cytoskeletal changes. |