4-Tubulin is involved in rapid formation of long microtubules to push apart the daughter centrosomes during early Drosophila embryogenesis

Although 4-tubulin comprises only about one-fifth of the -tubulin pool in every Drosophila egg, in the absence of 4-tubulin - in eggs of the kavar⁰/- hemizygous females - only a tassel of short microtubules forms with two barely separated daughter centrosomes. We report that 4-tubulin is enriched in the long microtubules that embrace the nuclear envelope and suggest that they push apart daughter centrosomes along the nuclear perimeter during the initial cleavage divisions. In vitro tubulin polymerization showed that 4-tubulin is required for rapid tubulin polymerization. Since tubulin polymerization is slow inside eggs of the kavar⁰/- females, only short microtubules can form within the 4 to 5 minutes allowed for the process. A tassel of short microtubules with two barely separated centrosomes forms in every egg of the Kavar^18c/+ females, in which the cytoplasm contains both wild-type and Kavar^18c-encoded 4-tubulin with an E82K amino acid substitution (E82K-4-tubulin). E82K-4-tubulin is incorporated into the microtubules and renders them unstable. When injected into wild-type early cleavage embryos E82K-4-tubulin slows down the formation of long microtubules and the separation of the daughter centrosomes. Surprisingly, when injected into late cleavage embryos E82K-4-tubulin is non-toxic. Similarly, in the neuroblasts, ectopically expressed E82K-4-tubulin becomes incorporated into the microtubules that grow sufficiently long and function normally.