Drosophila oocyte polarity and cytoskeleton organization require regulation of Ik2 activity by Spn-F and Javelin-like.

The Drosophila melanogaster Spn-F, Ik2, and Javelin-like (Jvl) proteins interact to regulate oocyte mRNA localization and cytoskeleton organization. However, the mechanism by which these proteins interact remains unclear. Using antibodies to activated Ik2, we showed that this protein is found at the region of oocyte and follicle cell where microtubule minus ends are enriched. We demonstrate that germ line Ik2 activation is diminished both in jvl and in spn-F mutant ovaries. Structure-function analysis of Spn-F revealed that the C-terminal end is critical for protein function, since it alone was able to rescue spn-F sterility. On the other hand, germ line expression of Spn-F lacking its conserved C-terminal region (Spn-FΔC) phenocopied ik2, leading to production of ventralized eggshell and bicaudal embryos. In Spn-FΔC-expressing oocytes, Gurken protein is mislocalized and oskar mRNA and protein localization is disrupted. Expression of Ik2 rescued Spn-FΔC ovarian phenotypes. We found that whereas Spn-F physically interacts with Ik2 and Jvl, Spn-FΔC physically interacts with Ik2 but not with Jvl. Thus, expression of Spn-FΔC, which lacks the Jvl-interacting domain, probably interferes with interaction of Ik2 and Jvl. In summary, our results demonstrate that Spn-F mediates the interaction between Ik2 and Jvl to control Ik2 activity.