Microtubule crossbridging by chlamydomonas dynein

Dynein, obtained from axonemes of Chlamydomonas, binds by both its A and B ends to microtubules assembled from twice cycled (2X) and purified (6S) brain tubulin as well as to microtubules in native spindles, thereby inducing microtubule crossbridging. The two ends of the dynein arm exhibit distinct binding characteristics for the different microtubule preparations. Greater than 99% of the dynein arms are bound exclusively by their B ends to microtubules assembled from 6S tubulin in the presence of dynein and decorated to saturation. In contrast, greater than 80% of the dynein arms are bound by both their A and B ends to and, therefore, crossbridge 6S microtubules that are only partially dynein decorated. Binding of the A end of the dynein arm to saturated 6S microtubules can be enhanced by destabilizing the binding of the B end upon addition of ATP and vanadate. These observations suggest that Chlamydomonas dynein arms can bind by their A ends to microtubules assembled from 6S tubulin only when the B ends of the arms either are not bound or are bound but do not occupy all available dynein binding sites. Dynein exhibits a slight preference for binding by its A end to microtubules assembled from 2X tubulin and containing microtubule associated proteins (MAPs). Approximately 90% of the dynein arms crossbridge adjacent 2X microtubules that are only partially decorated. But as saturation of these microtubules with dynein is approached, the majority of the arms are bound solely by their A ends, while a smaller percentage are bound by their B ends or by both their A and B ends. These studies indicate that the type of microtubule as well as the degree of saturation of the microtubule with dynein can determine whether microtubule crossbridging occurs.