EB-SUN, a New Microtubule Plus-End Tracking Protein in Drosophila.

Microtubule (MT) regulation is essential for oocyte development. In <Drosophila, MT stability, polarity, abundance, and orientation undergo dynamic changes across developmental stages. In our effort to identify novel microtubule-associated proteins (MAPs) that regulate MTs in the Drosophila ovary, we identified a previously uncharacterized gene, CG18190, encoding a novel MT end-binding (EB) protein, which we propose to name EB-SUN. We show that EB-SUN colocalizes with EB1 at growing microtubule plus-ends in Drosophila S2 cells. Tissue-specific and developmental expression profiles from Paralog Explorer reveal that EB-SUN is predominantly expressed in the ovary and early embryos, while EB1 is ubiquitously expressed. Furthermore, as early as oocyte determination, EB-SUN comets are highly concentrated in oocytes during oogenesis. EB-SUN knockout (KO) results in a decrease in MT density at the onset of mid-oogenesis (Stage 7) and delays oocyte growth during late mid-oogenesis (Stage 9). Combining EB-SUN KO with EB1 knockdown (KD) in germ cells significantly further reduced MT density at Stage 7. Hatching assays of single protein depletion reveal distinct roles for EB-SUN and EB1 in early embryogenesis, likely due to differences in expression and binding partners. Notably, all eggs from EB-SUN KO/EB1 KD females fail to hatch, suggesting partial redundancy between these proteins.