The karyopherin (Kap) family of nuclear transport factors facilitates 
macromolecular transport through nuclear pore complexes (NPCs). The binding of 
Kaps to their cargos can also regulate, both temporally and spatially, the 
interactions of the cargo protein with interacting partners. Here, we show that 
the essential yeast Kap, Kap121, binds Dam1 and Duo1, components of the 
microtubule (MT)-associated Dam1 complex required for linking dynamic MT ends 
with kinetochores (KTs). Like mutations in the Dam1 complex, loss of Kap121 
function compromises the formation of normal KT-MT attachments during mitosis. 
We show that the stability of the Dam1 complex in vivo is dependent on its 
association with Kap121. Furthermore, we show that the Kap121/Duo1 complex is 
maintained in the presence of RanGTP but Kap121 is released by the cooperative 
actions of RanGTP and tubulin. We propose that Kap121 stabilizes the Dam1 
complex and participates in escorting it to spindle MTs.