Investigation of the role of VML in the establishment of Drosophila embryonic dorsal-ventral polarity
Drosophila embryonic dorsal-ventral (D-V) polarity is defined by the expression of the gene pipe in the ventral cells of the follicular epithelium surrounding the developing oocyte. pipe encodes a sulfotransferase that transfers sulfate groups to several protein components of the vitelline membrane layer of the eggshell, including Vitelline Membrane-Like protein (VML). These sulfated proteins represent a ventral cue embedded in the eggshell, which, during embryogenesis, leads to the spatially-restricted activation of a serine protease cascade involving Gastrulation Defective (GD), Snake and Easter. Several important pieces of information missing from our understanding of Drosophila D-V patterning include the structures of the carbohydrates borne by the Pipe targets that represent the direct substrates for Pipe and how the Pipe-sulfated ventral cue triggers the spatially-regulated activation of the serine protease cascade. Two major goals of my studies on VML are to elucidate the structures of Pipe-sulfated carbohydrates associated with VML and to identify proteins that interact with VML in a Pipe-dependent manner. To achieve the first goal, I explored different purification systems to isolate VML and eventually found a way of partially purifying VML from Drosophila ovaries. Mass spectrometric analysis of the purified VML is underway to determine the carbohydrate structures on VML and the sites of Pipe-mediated sulfation. Future experiments will involve identification of putative enzymes responsible for the glycosylation of VML and examination of their requirements in D-V patterning. To identify interacting partners of VML, a strategy combining in vivo biotinylation of VML, reversible protein crosslinking and Streptavidin purification of crosslinked complexes will be used. In the second section of my studies, I have demonstrated that VML bearing a biotin acceptor peptide (BAP) tag can be efficiently biotinylated in vivo by co-expressing it with a biotin protein ligase BirA in the follicle cells. As an extension of the application of the approach, I also show that the Torso-like protein, which is localized to the poles of the vitelline membrane and whose polar localization is crucial for Drosophila terminal patterning, can also be biotinylated in vivo when the BAP-tagged protein is co-expressed with BirA in the follicle cells.