During Drosophila development, the follicular epithelium covering the developing oocyte undergoes morphological changes such that columnar epithelial cells that are adherent to each other at Stage 10A separate basally at Stage 10B, exposing regions of basolateral membrane to the underlying extracellular matrix. Schotman et al. found that, whereas the Golgi protein dGRASP localized to tER-Golgi units [where transitional endoplasmic reticulum (tER) is in proximity to the Golgi] in follicle cells at Stages 9 to 10A, it localized to the region of cell separation [the open zone of contact (ZOC)] at Stage 10B. dGRASP at the open ZOC did not colocalize with Golgi markers and was associated with the plasma membrane. Fluorescence in situ hybridization (FISH) revealed dgrasp mRNA also localized to the open ZOC, where double-labeling indicated that it colocalized with dGRASP protein. dgrasp mutation was associated with disruption of the follicular epithelium and failure of the integrin αPS1 subunit to localize to the open ZOC of Stage 10B follicle cells. Wing development (which depends on integrin-mediated epithelial adhesion) was also disrupted. FISH analysis indicated that αPS1 at the open ZOC was newly synthesized from mRNA. A combination of pharmacological and mutational analysis indicated that disruption of protein transport to and through the Golgi failed to block deposition of αPS1 to the open ZOC. Visualization of proteins involved in the exocytic pathway or their encoding mRNAs, or both, suggested that a subset localized with dGRASP. The authors conclude that a developmentally regulated unconventional dGRASP-dependent secretory pathway plays an important role in epithelial remodeling.
H. Schotman, L. Karhinen, C. Rabouille, dGRASP-mediated noncanonical integrin secretion is required for Drosophila epithelial remodeling. Dev. Cell 14, 171-182 (2008). [PubMed]