Cryo-electron microscopy unravels microtubule inner protein networks reinforcing mammalian sperm flagella

To find and fuse with the egg, mammalian sperm must complete an arduous voyage through the female reproductive tract. This odyssey is powered by the sperm tail, a specialized motile cilium. Mammalian sperm tails are reinforced at the molecular scale with sperm-specific microtubule inner proteins (sperm-MIPs), but the identities of these sperm-MIPs are unknown. Here, we report high-resolution cryo-electron microscopy structures of bovine sperm doublet microtubules (DMTs), allowing us to identify many sperm-MIPs. We also resolve structures of singlet MTs in the endpiece, revealing MIPs shared between singlet and doublet MTs. We demonstrate that at least two sperm-MIPs bind and stabilize MTs in vitro. Our structures shed light on ciliary diversity across cell types and provide structural frameworks for understanding molecular underpinnings of male infertility