The Dam1 Ring Binds Microtubules Strongly Enough To Be A Processive As Well As Energy-efficient Coupler For Chromosome Motion

Abstract

Accurate chromosome segregation during mitotic division of budding yeast depends on the multiprotein kinetochore complex, Dam1 (aka DASH). Purified Dam1 heterodecamers encircle microtubules (MTs) to form rings that can function as “couplers”, molecular devices that transduce energy from MT disassembly into the motion of a cargo. Here we show that MT depolymerization develops the 6-fold larger force against a Dam1 ring than the force exerted on a coupler that binds only one side of a MT. Wild type rings slow depolymerization 4-fold, but rings that include a mutant Dam1p with truncated C-terminus slow depolymerization less, consistent with the idea that this tail is part of a strong bond between rings and MTs. A molecular-mechanical model for Dam1-MT interaction predicts that binding between this flexible tail and the MT wall should cause a Dam1 ring to wobble as is moves with a shortening MT end, and Fourier analysis of moving, ring-attached beads corroborates this prediction. Comparisons of the forces generated against wild type and mutant complexes confirm the importance of tight Dam1-MT association for processive cargo movement under load. Since processivity in chromosome motion is an essential property, particularly for organisms in which there is only one MT per kinetochore, the Dam1 ring seems well adapted for mitotic motions in budding yeast.