AFM force measurements on microtubule-associated proteins: the projection domain exerts a long-range repulsive force

Microtubule-associated proteins (MAPs) are thought to control spacing betwe en microtubules. We propose that the projection domain is largely unstructu red and exerts a long-range repulsive force that is predominantly entropic in origin, providing a physical mechanism for maintaining spacing. To test this hypothesis, we developed an experimental system where MAN are electros tatically end-attached to a flat surface, such that the projection domains extend away from the surface. Atomic force microscopy force measurements on this system show that projection domains exert a long-range (> 100 nm) rep ulsive force. This force depends on the ionic strength of the solution in a way that is consistent with a polyelectrolyte polymer brush. (C) 2001 Publ ished by Elsevier Science B.V. on behalf of the Federation of European Bioc hemical Societies.