Freeze-Fracture of Microtubules and Bridges in Motile Axostyles

Abstract

A freeze-fracture study of the motile axostyles of the flagellate protozoa Saccinobaculus and Pyrsonympha has been undertaken in order to obtain a view of the relationships of microtubules and their cross bridges not dependent on conventional preparative procedures. Reactivation studies using isolated axostyles prepared for freeze-fracture and then thawed demonstrate that we are observing the structure of a potentially functional axostyle. Cross fractures through the axostyle demonstrate more extensive interrow bridging than expected on the basis of observations of thin-sectioned material. Each microtubule has approximately sixfold bridge-binding sites with connections to as many as four interrow bridges. Measurements of microtubule diameter and spacing are significantly larger than those made from sectioned material and may indicate that conventional processing for electron microscopy results in the loss of structurally important water within the microtubule in addition to loss of intertubule material. Longitudinal fractures through the axostyle at various orientations demonstrate a minimum longitudinal periodicity of 160 A for both the spacing of the globular subunits within the microtubule wall and the spacing of the intrarow bridges. While intrarow bridges are strictly periodic and always oriented in parallel, interrow bridges are not strictly periodic and can be oriented at varying angles to the microtubule axis.

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

“…We have observed the tilt of the cross-bridges in both rapid-frozen live protozoa and axostyles isolated in the presence of ATP. In agreement with Bloodgood and Miller (6) and McIntosh (26), cross-bridges between two interacting microtubules all appear to have the same angle of tilt along a given length of microtubule (along 0.5 pin or^-30 cross-bridge repeats; Fig. 5 a, c, and e), but the angle of these groups of cross-bridges varies from region to region within the same speciment (Fig.…”

Symbiotic Innovation in the Oxymonad Streblomastix strix

“…The central core of S. strix is almost entirely filled by an axostyle that encircles a long thread‐like nucleus, a configuration that is likely a structural constraint of streamlining. The axostyle of most oxymonads is quite robust and motile (Bloodgood and Miller 1974; Brugerolle and Konig 1997; McIntosh 1973). However, the axostyle of S. strix is very reduced and almost completely rigid, which certainly helps stabilize the attached bacteria on the cell surface.…”

“…A thin veil of microtubules, called the ‘pelta’, usually arises from one of the basal bodies, supports the anterior surface of the cell and surrounds the anterior region of the nucleus. The flagella may be oriented posteriorly or anteriorly, and in the former case may be physically attached to the cell surface (Bloodgood et al 1974; Hollande and Carruette‐Valentin 1970). …”

A motile system of singlet microtubules in spermatozoa

“…It could therefore correspond to the Gibbons'"Dynein 1" [Gibbons et al, 19761. This system seems to be quite similar to the protozoan axostyle, where single tubules are also linked by arms [ McIntosh, 1973McIntosh, , 1974Tilney and Byers, 1969;Harris, 1970;Bloodgood and Miller, 1974;Woodrum and Linck, 19801 and where a dynein band is present [Mooseker and Tilney, 19731. One difference is that in axostyle the waves go from the anterior to the posterior extremity of the cell, whereas in coccid spermatozoa the direction is from posterior to anterior.…”

“…The best known model is a protozoan motile organelle, the axostyle, consisting of a ribbon of microtubules oriented longitudinally, interconnected by linkages [McIntosh, 1973[McIntosh, , 1974Tilney and Byers, 1969;Harris, 1970;Bloodgood and Miller, 1974;Woodrum and Linck, 19801. This system is vigorously motile by planar waves propagating from the anterior of the cell to the posterior [Bloodgood and Fitzharris, 1978;Langford and InouC, 19791, shows ATPase activity between microtubules and an electrophoretic band in the region of the dynein polypeptides [Mooseker and Tilney, 19731, and is influenced by magnesium [Bloodgood, 19751 and by calcium [Schliwa, 19771.…”