Microtubules, the ER and Exu: new associations revealed by analysis of mini spindles mutations.

During Drosophila oogenesis, organized microtubule networks coordinate the localization of specific RNAs, the positioning of the oocyte nucleus, and ooplasmic streaming events. We used mutations in mini spindles (msps), a microtubule-associated protein, to disrupt microtubule function during mid- and late-oogenesis, and show that msps is required for these microtubule-based events. Since endoplasmic reticulum (ER) organization is influenced by microtubules in other systems, we hypothesized that using msps to alter microtubule dynamics might affect the structure and organization of the ER in nurse cells and the oocyte. ER organization was monitored using GFP-tagged versions of Reticulon-like1 and protein disulfide isomerase. Analyses of living cells indicate microtubule associations mediate the movement of ER components within the oocyte. Surprisingly, the distribution and behavior of tubular ER in the oocyte differs from general ER, suggesting these two compartments of the ER interact differently with microtubules. We find that the morphology of Exu particles is msps-dependent, and that Exu is specifically associated with tubular ER in msps mutants. Our results extend previous descriptions of sponge bodies and the fusome, suggesting both are manifestations of a dynamic structure that interacts with microtubules and persists throughout oogenesis.