with weak Dam1–MT binding that per mits ring diffusion. How-
ever, these movements were seen in the absence of soluble
bead-f ree Dam1, and it remains to be determined whether such
c oupling is ring-based.
Our model defines an inverse relationship between the effi-
cienc y of force transduction and the stabilit y of c oupler attach-
ment, leav ing a relatively narrow window of ‘‘successful’’
ring–MT bond energies, i.e., those in which c oupler performance
is optimal by the postulated standards (Fig. 3D, shaded area). A
c ompromise bet ween reduced efficiency of force transduction
and increased strength of attachment appears important for the
c oupler in an organism like Saccharomyces cerevisiae, where a
k inetochore is st ably attached to only one MT (41, 42). We
therefore favor a tight-binding mechanism with k
T and consider it better suites for budding yeast than the one
based on biased diffusion. However, a choice between different
c oupler designs is impossible on strictly theoretical grounds. For
example, although the forces that oppose kinetochore motions
may be large in some organisms (43), in budding yeast, their size
has not been measured.
Experimental Approaches to Study the Mechanism of Dam1 Motility.
If the Dam1 ring is similar to our theoretical ring, our results
suggest that its mechanism of motility in vitro can be determined
by characterizing ring motion at a depolymerizing MT end. If the
ring moves by biased diffusion, the MT end serves only to ratchet
the ring’s random walk. Thus, the coupler’s speed will be
deter mined simply by the rate of MT depoly merization (Fig. 4D).
The same result is expected in an electrostatic model (33), as well
as in any other model in which the ring slides freely on the MT
surface (37). If the ring binds strongly, however, it will reduce the
rate of MT disassembly. A reduced rate of PF splitting has been
verified experimentally for the force-transducing couplers under
opposing tension (13, 44). Examination of the Dam1 ring’s
motilit y in the absence of a load, however, is lacking. Additional
infor mation will be available from an analysis of mutant Dam1
c omplexes with reduced MT binding (29). If strong ring–MT
binding is facilitated by linkers, complexes with reduced binding
linker should slow MT depolymerization less than wild type. If,
however, these linkers serve other purposes, e.g., if they facilit ate
the sliding (32, 37), the above mutant should retard MT disas-
sembly more than wild type. Ex perimental testing of these
predictions should help clarify the mechanism of Dam1 ring
We thank M. Molodtsov, N. Goudimchuk, and other members of the
McIntosh and At aullakhanov laboratories for help and assistance and
A. I. Vorobjev for support. We are also grateful to D. Dr ubin, G. Oster,
K. Bloom, J. Scholey, and D. Odde for discussions. This work was
supported by National Institutes of Health Grant GM33787 (to J.R.M.)
and U.S. Civ ilian Research and Development Foundation Grant
CGP2006B#2863 (to F.I.A. and J.R.M.).
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