Cell migration is an important embryological process that underlies tissue specification and differentiation. During Drosophila gastrulation,the fibroblast growth factor receptor Heartless (HTL) is required for mesoderm migration. Now, Schumacher et al.() and Smallhorn et al. () report that Pebble (PBL), a guanyl nucleotide exchange factor for RHO1, is also required for this process, independent of its known function in cytokinesis. Schumacher and colleagues describe the shape changes that mesoderm cells undergo during migration, and show that HTL is required for their protrusive activity and their adhesion to the ectoderm. Then, in a genetic screen they identify pbl as a new gene required for mesoderm migration and report that constitutively active HTL cannot rescue the early shape change defects seen in pbl mutants, indicating that PBL is involved in HTL-triggered cell migration in Drosophila gastrula. In their paper, Smallhorn and co-workers report that pbl mutant mesodermal cells are more tightly adherent to their neighbours than are wild-type cells, and that they fail to undergo the normal epithelial-mesenchymal transition. These researchers also show that the GTP exchange function of PBL is required for this transition.
