Microtubule biopolymers

A rather spectacular effect of radiation damage is shown in Figure 9. This example shows aligned rigid rod microtubule biopolymers in solution. Not only... 

Fig. 9. An initially aligned sample of rigid rod microtubule biopolymers in solution after it has been hit by radiation. The sample is viewed between crossed polarizers. The burn mark is centrally placed. The alternating dark and light bands are domains that have a different orientation. The transition from uniform alignment to domains with different orientation takes place in seconds after the sample is irradiated. The burn mark has dimensions of roughly 0.5 x 3 mm. The shift of the domains with respect to each other is approximately 0.5 mm as witnessed by the distortions of the bum mark.

Fig. 24. Diffraction pattern of a magnetically aligned sample of microtubule biopolymers. The orientation was achieved by assembling the poljrmers, from the tubulin protein solution, in a 9 T on-line magnet. The diamagnetic moment of the tubulin protein is sufficiently high to induce orientation once several tubulin dimers have assembled. Unpublished data, courtesy of F. Diaz, G.R. Diakun, and W. Bras.

Scattering from Magnetically Oriented Microtubule Biopolymers... 

Mandelkow, E., Lange, G., Mandelkow, E.-M. Applications of Synchrotron Radiation to the Study of Biopolymers in Solution Time-Resolved X-Ray Scattering of Microtubule Self-Assembly and Oscillations. 151, 9-29 (1989). 

The above theory can be extended to deal with other more complex cases. For example, the two ends of a biopolymer need not behave identically, and microtubules, as noted earlier, are helical polymers of asymmetric protomer units. Thus, two sets of on- and off-constants might be necessary. In other cases, such as in the polymerization of tubulin in the presence of tubulin—colchicine complex (Sternlicht and Ringel, 1979 Sternlicht et al., 1980), there may be the need to consider copolymerization possibilities. 

The above theory allows one to estimate the off-rate constant for biopolymer disassembly, but it does not take into account the possibility of having distinctly different rate constants at each end of the polymer. Thus, one obtains the sum of the off-rate constants rather than the exact constants for each end. Another limitation is that one does not know how many growing points there are at each microtubule end, and from the discussion of the multiplicity of helical starts (Amos et al., 1976), the observed off-constant should represent the product of n (the number of growing points) and the actual microscopic rate constant for each of these growing points. As will be seen below, axoneme-promoted assembly of microtubules (Bergen and Borisy, 1980) may help to obviate the former limitation, but the latter requires further characterization of the true growing points. 

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