Alternative splicing repurposes the Drosophila mitotic regulator Mud for meiotic functions

Mud/NuMA/LIN-5 (in flies, vertebrates, and worms) is an evolutionarily conserved protein that regulates the shape and orientation of the mitotic spindle. In vertebrate cells, these functions depend on a C-terminal region called the NuMA-Tip, which (i) mediates interaction with the conserved partner protein LGN (called Pins in flies), (ii) contains a highly conserved subsequence called the NLM, and (ii) binds directly to microtubule ends. Although Mud plays a vital role in Drosophila mitosis, less is known about its structure, particularly at the C-terminus. Through sequence analysis and functional studies, we identify the Mud-Tip region and show that it is encoded by 3 consecutive exons. These exons are spliced out of several Mud isoforms, creating functionally distinct "Tipless" variants. We find that Tipless isoforms are specifically expressed in male and female gametes, where they localize to the nuclear envelope. Although Mud is known to be essential for female fertility, we demonstrate that this function does not require an intact Tip region. We also find that Mud antagonizes the localization of Lamin, a nucleoskeletal protein, in the testis, and uncover an unexpected role for Tipless Mud in promoting male fertility. Our work reveals that while the Mud-Tip is important for Mud function at mitosis, alternative splicing ensures this region is absent from Mud isoforms that perform a moonlighting role during meiosis.