Structural basis of motility in the microtubular axostyle: implications for cytoplasmic microtubule structure and function.

Abstract

The gross morphology of the protozoan microtubule axostyle of Saccinobaculus ambloaxostylus can now be described in macromolecular detail. The left-handed coil of the axostyle is seen to be dependent upon the asymmetry inherent in the constituent microtubules as expressed by the specific array of linkages between microtubules and by a possible tendency for microtubules to coil into left-handed helices. The laminated sheets of microtubules are not aligned parallel to the long axis of the organelle, but become increasingly tilted off-axis as one descends through the sheets of microtubules from the convex to the concave surface of the axostyle. Fine-structural analysis of the axostyle indicates similarities of the linkages to dynein. The potential loci of the force-generating protein(s) are discussed as well as implications of the axostyle's structure on general microtubule function.

Different structural states of a microtubule cross-linking molecule, captured by quick-freezing motile axostyles in protozoa.

“…Successful reactivation has generally been obtained only when detergent extraction and ATP exposure are performed in rapid succession directly on the microscope stage, without washing away soluble cellular contents or subjecting axostyles to centrifugation (2,29,40). In fact, the only axostyles in our preparations that show rhythmic, propagated activation upon exposure to ATP are ones that attach to the glass slide during viewing; these happen to retain their nucleus and other cytoplasmic components.…”

“…This somehow generates bends perpendicular to the long axis of the whole ribbon (3, 8, 17, 24-26, 29, 34), and because the axostyle is permanently twisted along its length, it also involves a change in helical pitch. Additionally, the bends propagate from one end of the axostyle to the other, causing the organelle to rotate inside the cell as well as to undulate.Isolated axostyles can regenerate this movement in vitro when exposed to ATP in an appropriate ionic environment (2,29,40). The challenge is thus to explain how molecular ATPases that presumably reside in the cross-bridges between microtubules can affect the helical pitch of the overall lattice.…”

“…Isolated axostyles can regenerate this movement in vitro when exposed to ATP in an appropriate ionic environment (2,29,40). The challenge is thus to explain how molecular ATPases that presumably reside in the cross-bridges between microtubules can affect the helical pitch of the overall lattice.…”

“…The challenge is thus to explain how molecular ATPases that presumably reside in the cross-bridges between microtubules can affect the helical pitch of the overall lattice. This is generally thought to involve some sort of sliding of microtubules relative to each other (2,17,26,29,40), analogous to the dynein-generated sliding of microtubules in cilia and flagellae (33); but the geometrical complexity of the axostyle has thwarted efforts to specify exactly which crossbridges are involved and what sort of sliding occurs.A previous report by Woodrum and Linck (40) provided the first deep-etch images of axostyle cross-bridges. These turn out to look sufficiently similar to dynein cross-bridges in deep-etched ciliary and flagellar axonemes (11-14) to suggest a functional homology.…”

Unlocking the secrets of multi-flagellated propulsion: drawing insights fromTritrichomonas foetus

“…Previous studies have revealed that the cytoskeleton (i.e. axostyle and costa) of T. foetus remains rigid throughout the entirety of its swimming motion [24], as opposed to other trichomonads [25] and Giardia [14], in which body flexion can be observed. Because the cell body of T. foetus is shown to be rigid during swimming, the propulsive forces necessary for swimming must be the result of flagellar beating [24], making this microorganism ideally suited for the study of multi-flagellated propulsion.…”