A new family of putatively heavily glycosylated extracellular proteins contribute to both eggshell stability and differentation of the terminal poles of embryos in Drosophila. Jiménez et al. cloned and characterized the proteins encoded by the genes fs(1)Nasrat and fs(1)polehole. These two proteins are leucine-rich and contain as many as 25 glycosylation sites and several glycosanimoglycan sites. Localization in developing oocytes is extracellular, and the proteins appear to be synthesized by the nurse cells and developing oocytes. Loss of either protein (in mutant flies) resulted in loss of both proteins, suggesting that the proteins interact to stabilize each other. Using mutants that disrupted only terminal pattern formation and not eggshell stability, the authors showed that loss of either Nasrat or Polehole resulted in a severe loss of extracellular Torso-like. Torso-like promotes the cleavage of Trunk at the oocyte poles to release the active Trunk ligand, which initiates signaling by the receptor tyrosine kinase, Torso. The authors propose that the localized distribution of Nasrat and Polehole allows the localized stabilization or accumulation of the secreted factors involved in pattern formation during oogenesis.
G. Jiménez, A. González-Reyes, J. Casanova, Cell surface proteins Nasrat and Polehole stabilize the Torso-like extracellular determinant in Drosophila oogenesis. Genes Dev.. 16, 913-918 (2002). [Abstract] [Full Text]