Characterization of Xenopus RalB and its involvement in F-actin control during early development

We describe the characterization and a functional analysis in Xenopus devel opment of RalB, a small G protein. RalB RNA and protein are detectable duri ng oogenesis and early development, but the gene is expressed only weakly i n adult tissues. The RalB transcripts are processed by poly(A) extension du ring oocyte maturation and up to the gastrulation stage. Microinjection of wild-type or mutant RalB RNAs was performed in fertilized eggs in order to gain insight into the function of RalB during development. We show that dur ing cleavage stages the activated GTP form of RalB specifically induces a c ortical reaction that affects the localization of pigment granules. The use of different drugs suggests that this reaction is dependent on the outer c ortical actin array. The relation between E-actin and RalB was shown by con focal analysis. Injection of mRNAs encoding the mutated activated form of R alB leads, at dependent doses, to a blocking of gastrulation or defects in closing of neural folding structures. In contrast, the inactivated form blo cks only the closing of neural tube. Altogether, these observations suggest that RalB is part of a regulatory pathway that may affect the blastomere c ytoskeleton and take part in early development, (C) 1999 Academic Press.