Conoids consist of tubulin in an unconventional arrangement.

The pathogens that cause malaria, cryptosporidiosis, and toxoplasmosis share a baffling common feature: a cone-shaped apical structure called the conoid. Previous research has suggested that the conoid is composed of microtubules and has simultaneously provided evidence that it is not. Now, on page 1039, Hu et al. resolve this longstanding dilemma by demonstrating that the conoid is made of tubulin, but not in the form of classical microtubules. In addition to describing a previously unknown form of cytoskeletal structure, the results could help in the development of new treatments for some of the world's most devastating diseases.

The conoid is part of the apical complex, the defining feature of the phylum Apicomplexa and a structure thought to be involved in host cell invasion by these parasites. Although tubulin was considered the most likely building block of the conoid, the bends in the structure appeared too tight to be accommodated by microtubules. Combining transgenic Toxoplasma gondii and an array of imaging techniques, the authors confirmed that the conoid is composed of tubulin protofilaments, supporting the microtubule hypothesis. But careful ultrastructural analysis shows that the tubulin in the conoid assembles in a manner never before observed, creating a structure with a comma-shaped cross section rather than a microtubule. Previously described tubulin structures are generally circular or semicircular in cross section.Hu et al. also found that the conoid fibers are assembled rapidly during the early phases of cell division. The flattened cross section explains how tubulin can conform to the tight bends found in the conoid—a ribbon is easier to bend than a straw—but more work will be needed to understand how the cell directs the assembly of this novel structure, and how the increased pitch of the fibers and the translation of the entire structure contribute to the spring-like action of the conoid. The conoid's unique architecture makes it an attractive drug target, especially since parasite and host microtubules are known to differ in their sensitivity to various compounds. ▪