Unraveling the intricate microtubule inner protein networks that reinforce mammalian sperm flagella

To find and fuse with the egg, mammalian sperm must complete an arduous voyage through the female reproductive tract. The sperm cell’s remarkable odyssey is powered by its flagellum, a microtubule-based molecular machine ornamented with accessory structures that stabilize the sperm tail in viscous media. Recently, cryo-electron tomography (cryo-ET) revealed that mammalian sperm flagella are further reinforced at the molecular scale with sperm-specific microtubule inner proteins (sperm-MIPs), but the identities of these sperm-MIPs are unknown. Here, we use cryo-electron microscopy to resolve structures of native bovine sperm doublet microtubules, thus identifying most sperm-MIPs. In the A-tubule, several copies of testis-specific Tektin-5 contribute to an extended protein network spanning nearly the entire microtubule lumen. Different copies of Tektin-5 adopt a range of conformations and organizations based on their local interactions with other MIPs. The B-tubule is in turn stabilized by sperm-MIPs that bind longitudinally along and laterally across protofilaments. We further resolve structures of endpiece singlet microtubules, revealing MIPs shared between singlets and doublets. Our structures shed light on the molecular diversity of cilia across different cell types of the vertebrate body and provide a structural framework for understanding the molecular underpinnings of male infertility. ### Competing Interest Statement The authors have declared no competing interest.