Evidence for actin filament-microtubule interaction mediated by microtubule-associated proteins

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Abstract
Low shear viscometry and EM was used to study the interaction between pure actin filaments from rabbit shielded muscle and microtubules. Mixtures of microtubules having microtubule-associated proteins (MAPs) from calf brain gray matter with actin filaments, have very high viscosities compared with the viscosities of the separate components. MAP themselves also cause a large increase in the viscosity of actin filaments. In contrast, mixtures of actin filaments with tubulin polymers lacking MAP have low viscosities, close to the sum of the viscosities of the separate components. There is an interaction between actin filaments and microtubules which requires MAP. This interaction is inhibited by ATP and some related compounds. EM of thin sections through mixtures of actin and microtubules show numerous close associations between the 2 polymers which may be responsible for their viscosity.