Embryogenesis requires an increase in total cellular membrane and the generation of distinct membrane domains. In the Drosophila embryo, this membrane growth and refinement process is called cellularization and results in a polarized epithelium. Lecuit et al. show that a 135-kD membrane-associated protein called Slam (slow-as-molasses) is required for basal-lateral membrane growth. The slam mutant embryos were disrupted at gastrulation because of aberrant furrow canal formation and membrane invagination. As a result, no basal-lateral region distinct from the apical membrane was formed, nor were junctions between adjacent plasma membranes. Ectopically expressed Slam localized to the plasma membrane of the furrow canal and to the adjacent basal-lateral membrane during the first, slow phase of embryonic cellularization. Slam expression decreased in the later, fast phase of cellularization just before gastrulation. The transport of two membrane proteins, neurotactin and Toll, was also inhibited in slam mutants. Slam colocalized with Discs-lost and Armadillo, two proteins required for maintaining cell junctions. Myosin localization to these junctions was compromised in slam mutants. The authors propose that Slam could induce the formation of protein complexes that connect the actin-myosin cytoskelton to distinct regions of membrane growth and further stabilize or enhance functional insertion of membranes basal-laterally during the process of cell polarization.
T. Lecuit, R. Samanta, E. Wieschaus, slam encodes a developmental regulator of polarized membrane growth during cleavage of the Drosophila embyro. Dev. Cell 2, 425-436 (2002). [Online Journal]