Spire contains actin binding domains and is related to ascidian posterior end mark-5

Spire is a maternal effect locus that affects both the dorsal-ventral and a nterior-posterior axes of the Drosophila egg and embryo. It is required for localization of determinants within the developing oocyte to the posterior pole and to the dorsal anterior corner. During mid-oogenesis, spire mutant s display premature microtubule-dependent cytoplasmic streaming, a phenotyp e that can be mimicked by pharmacological disruption of the actin cytoskele ton with cytochalasin D. Spire has been cloned by transposon tagging and is related to posterior end mark-5, a gene from sea squirts that encodes a po steriorly localized mRNA, Spire mRNA is not, however, localized to the post erior pole. SPIRE also contains two domains with similarity to the actin mo nomer-binding WH2 domain, and we demonstrate that SPIRE binds to actin in t he interaction trap system and in vitro. In addition, SPIRE interacts with the rho family GTPases RHOA, RAC1 and CDC42 in the interaction trap system. Thus, our evidence supports the model that SPIRE links rho family signalin g to the actin cytoskeleton.