Cryo-ET reveals the in situ architecture of the polar tube invasion apparatus from microsporidian parasites

Microsporidia are divergent fungal pathogens that employ a unique harpoon-like apparatus called the polar tube (PT) to invade host cells. The long PT is fired out of the microsporidian spore over the course of just a few hundred milliseconds. Once fired, the PT is thought to pierce the plasma membrane of a target cell and act as a conduit for the transfer of the parasite into the host cell, which initiates infection. The PT architecture and its association with neighboring organelles within the parasite cell remain poorly understood. Here, we use cryoelectron tomography to investigate the structural cell biology of the PT in dormant spores from the human-infecting microsporidian species, <i>Encephalitozoon intestinalis</i>. Segmentation and subtomogram averaging of the PT reveal at least four layers: two protein-based layers surrounded by a membrane layer and filled with a dense core. Regularly spaced protein filaments form the structural skeleton of the PT. Combining cryoelectron tomography with cellular modeling, we propose a model for the three-dimensional organization of the polaroplast, an organelle that surrounds the PT and is continuous with the outermost, membranous layer of the PT. Our results reveal the ultrastructure of the microsporidian invasion apparatus in situ, laying the foundation for understanding infection mechanisms.