Phosphoregulation and depolymerization-driven movement of the Dam1 complex do not require ring formation

During mitosis, kinetochores form persistent attachments to microtubule tips and undergo corrective detachment in response to phosphorylation by Ipl1 ( Aurora B) kinase(1). the Dam1 complex is required to establish and maintain bi-oriented attachment to microtubule tips in vivo, and it contains multiple sites phosphorylated by Ipl1 ( refs 2-10). Moreover, a number of kinetochore-like functions can be reconstituted in vitro with pure Dam1 complex(11-14). these functions are believed to derive from the ability of the complex to self-assemble into rings(12,13,15-17). Here we show that rings are not necessary for dynamic microtubule attachment, Ipl1-dependent modulation of microtubule affinity or the ability of Dam1 to move processively with disassembling microtubule tips. Using two fluorescence-based assays, we found that the complex exhibited a high affinity for microtubules ( K-d of approximately 6 nM) that was reduced by phosphorylation at ser 20, a single Ipl1 target residue in Dam1. Moreover, individual complexes underwent one-dimensional diffusion along microtubules and detached 2.5-fold more frequently after phosphorylation by Ipl1. Particles consisting of one to four Dam1 complexes - too few to surround a microtubule - were captured and carried by disassembling tips. thus, even a small number of binding elements could provide a dynamic, phosphoregulated microtubule attachment and thereby facilitate accurate chromosome segregation.