Microsporidia are a class of obligate intracellular parasitic unicellular eukaryotes that infect a variety of hosts, even including humans. Although different species of microsporidia differ in host range and specificity, they all share a similar infection organelle, the polar tube, which is also defined as the polar filament in mature spores. In response to the appropriate environmental stimulation, the spore germinates with the polar filament everted, forming a hollow polar tube, and then the infectious cargo is transported into host cells via the polar tube. Hence, the polar tube plays a key role in microsporidian infection. Here, we review the origin, structure, composition, function, and application of the microsporidian polar tube, focusing on the origin of the polar filament, the structural differences between the polar filament and polar tube, and the characteristics of polar tube proteins. Comparing the three-dimensional structure of PTP6 homologous proteins provides new insight for the screening of additional novel polar tube proteins with low sequence similarity in microsporidia. In addition, the interaction of the polar tube with the spore wall and the host are summarized to better understand the infection mechanism of microsporidia. Due to the specificity of polar tube proteins, they are also used as the target in the diagnosis and prevention of microsporidiosis. With the present findings, we propose a future study on the polar tube of microsporidia. Graphical Abstract

Chitin, an extracellular polysaccharide, is synthesized by membrane-embedded chitin synthase (CHS) utilizing intracellular substrates. The mechanism of the translocation of synthesized chitin across the membrane to extracellular locations remains unresolved. We prove that the chitin synthase from Phytophthora sojae (PsCHS) is a processive glycosyltransferase, which can rapidly produce and tightly bind with the highly polymerized chitin. We further demonstrate that PsCHS is a bifunctional enzyme, which is necessary and sufficient to translocate the synthesized chitin. PsCHS was purified and then reconstituted into proteoliposomes (PLs). The nascent chitin is generated and protected from chitinase degradation unless detergent solubilizes the PLs, showing that PsCHS translocates the newly produced chitin into the lumen of the PLs. We also attempted to resolve the PsCHS structure of the synthesized chitin-bound state, although it was not successful; the obtained high-resolution structure of the UDP/Mn²⁺-bound state could still assist in describing the characterization of the PsCHS’s transmembrane channel. Consistently, we demonstrate that PsCHS is indispensable and capable of translocating chitin in a process that is tightly coupled to chitin synthesis.

Microsporidia are obligate intracellular parasites that infect a wide variety of hosts, including humans. Microsporidian spores possess a unique, highly specialized invasion apparatus involving the polar filament, polaroplast and posterior vacuole. During spore germination, the polar filament is discharged out of the spore forming the hollow polar tube that transports the sporoplasm components including nucleus into the host cell to achieve the invasion. Due to the complicated topological changes occurring in this process, the formation of sporoplasm is unclear. Here, electron microscopy observation and DiI staining confirmed that during spore germination, a large number of vesicles derived from the polaroplast, nucleus and other cytoplasm were transported out via the polar tube. Meanwhile, the posterior vacuole and plasma membrane remained in the empty spore coat. In addition, there was no DiI-labeled membrane around the nucleus in mature spores, whereas a DiI-labeled limit membrane wrapping nucleus was found at the tip of the extruded polar tube, suggesting that the membrane of sporoplasm was formed outside the mature spore. Two Nosema bombycis sporoplasm surface proteins (NbTMP1 and NoboABCG1.1) were located at the polaroplast in mature spores, in the extruded polar tube and on the sporoplasm membrane, which indicated that the polaroplast transported via the polar tube finally became the limiting membrane of the sporoplasm. Golgi-tracker green and Golgi marker protein syntaxin 6 were also found the same model, which was consistent with the transported polaroplast derived from Golgi transformed into the novel sporoplasm membrane during spore germination. Importance: Microsporidia, obligate intracellular pathogenic organisms, cause huge economic losses in agriculture and even threaten human health. The key to successful infection of microsporidia is its unique invasion apparatus which includes the polar filament, polaroplast and posterior vacuole. When the spore is activated to geminate, the polar filament uncoils and undergoes a rapid transition into the hollow polar tube that will transport the sporoplasm components including nucleus into a host cell to achieve the invasion. Knowledge of structure difference between polar filament and polar tube, the process of cargo transport in extruded polar tube, and the formation of the sporoplasm membrane are still poorly understood. Herein, we verify that the polar filament evaginates to form the polar tube, which serves as a conduit for transporting elongated nucleus and other sporoplasm components. And we confirm that the transported polaroplast finally transforms into the novel sporoplasm membrane during spore germination. Our study provides new insights into the cargo transportation process of polar tube and origin of the sporoplasm membrane, which serve as foundations for clarifying the microsporidian infection mechanism.