Novel roles for BicD in pronuclear fusion and meiosis II progression via localization of the CHC/TACC/Msps complex to MII spindles

Vertebrate Clathrin heavy chain (Chc) plays a moonlighting function during mitosis. Chc forms a complex with TACC3 (Transforming Acidic Coiled Coil 3) and ch-TOG (colonic hepatic tumor overexpressed gene) at the spindle microtubules, forming inter microtubule bridges that stabilize the K-fibers. Since Drosophila Chc is a cargo of the dynein adaptor Bicaudal-D (BicD), we investigated whether BicD regulates Clathrin function at the spindle. We found that BicD localizes, like Chc, to centrosomes and spindles during mitosis and meiosis II, and that Chc interacts with Drosophila TACC (D-TACC). Using deGradFP to reduce the activity of BicD in mature eggs and very young embryos, we uncovered a novel function of BicD in meiosis II. The affected meiosis II products underwent abnormal rounds of additional replications and failed to carry out pronuclear fusion. Pointing to a mechanism, we found that the localization of Clathrin/D-TACC/Minispindles (Msps, homolog of ch-TOG) to the meiosis II spindles was impaired upon BicD knockdown. Furthermore, the meiotic products showed abnormal staining for PH3 and reduced recruitment of spindle assembly checkpoint (SAC) components. Altogether, our results support the notion that BicD performs a key activity in assembling the meiotic spindle apparatus. This function of BicD seems conserved in evolution because C. elegans embryos with reduced activities of these genes developed comparable phenotypes.