The Drosophila Ral GTPase regulates developmental cell shape changes through the jun NH2-terminal kinase pathway

The Ral GTPase is activated by RalGDS, which is one of the effector protein s for Pas. Previous studies have suggested that Ral might function to regul ate the cytoskeleton; however, its in vivo function is unknown. We have ide ntified a Drosophila homologue of Ral that is widely expressed during embry ogenesis and imaginal disc development. Two mutant Drosophila Ral (DRal) pr oteins, DRal(G20V) and DRal(S25N), were generated and analyzed for nucleoti de binding and GTPase activity. The biochemical analyses demonstrated that DRal(G20V) and DRal(S25N) act as constitutively active and dominant negativ e mutants, respectively. Overexpression of the wild-type DRal did not cause any visible phenotype, whereas DRalG20V and DRalS25N mutants caused defect s in the development of various tissues including the cuticular surface, wh ich is covered by parallel arrays of polarized structures such as hairs and sensory bristles. The dominant negative DRal protein caused defects in the development of hairs and bristles. These phenotypes were genetically suppr essed by loss of function mutations of hemipterous and basket, encoding Dro sophila Jun NH,terminal kinase kinase (JNKK) and Jun NH2-terminal kinase (J NK), respectively. Expression of the constitutively active DRal protein cau sed defects in the process of dorsal closure during embryogenesis and inhib ited the phosphorylation of JNK in cultured S2 cells. These results indicat e that DRal regulates developmental cell shape changes through the JNK path way.