Isolation of the cytoskeleton from Giardia. Tubulin and a low-molecular-weight protein associated with microribbon structures.

The sucking disk of Giardia is supported by a large, plate-like organelle: the ventral disk cytoskeleton. Extraction by Triton-X 100 of Giardia trophozoites from the mouse gut, or of G. duodenalis or G. lamblia grown from cultures, yields cell-free disk cytoskeletons. Up to 8 flagellar axonemes may be attached to an isolated disk. Disks are seen in the electron microscope to be composed of concentrically coiled microtubules bonded to microribbons. Microribbons are large, laminated structures, linked by dense networks of crossbridges. They are made up of regularly arranged subunits. Microtubules and microribbons are preserved in Triton for long periods, but crossbridges are slowly dissolved. Whereas the addition of ATP causes axonemes to resume bending, active movements were not detected in disks. It seems more likely that a disk is a passive effector, which may be acted upon by other contractile structures of the cytoplasm. It is highly specialized and quintessential cytoskeleton. Disks and axonemes will dissolve in sodium dodecyl sulphate (SDS), and after SDS-gel electrophoresis 2 prominent bands are apparent. One, corresponding to tubulin, migrates in low ionic strength, high pH buffers as 2 closely spaced bands of equal staining density. The other, a smaller protein, is a complex polypeptide band of molecular weight 30 000 Daltons. Allowing for staining differences, the 2 proteins are probably present in cytoskeletons in roughly equal amounts. Because of their size microribbons account for at least 75% of the structured material stained by electron stains in pellets of cytoskeletons. Disk and axoneme microtubules comprise the minority fraction. This result suggests that, like microtubules, microribbons are a source of structural tubulin and probably also contain the 30 000 mol. wt protein.