Figure 10.
The role of Bud6p and Kar9p in cortical capture. A model summarizing astral MT dynamic behavior associated with Bud6p or Kar9p during MT capture and establishment of spindle polarity. (1) As a cell proceeds through G1, the old SPB moves toward the incipient bud site by shrinking of astral MTs (thin black lines) at cortical Bud6p sites. (2) After bud emergence and SPB duplication, Kar9p or Bud6p-associated capture coexist and can separately direct astral MTs to the bud cortex. For example, in 2a, an MT reaching cortical Bud6p undergoes shrinkage coupled with SPB movement toward the cortex (without Kar9p being present at the MT plus end); in 2b, Kar9p translocates to an MT plus end and binds to Myo2p to guide the MT to the bud via an actin cable. In addition, Kar9p can travel along astral MTs already contacting the cortex (at Bud6p). Kar9p may also prevent MTs in the bud from undergoing catastrophe past the bud neck. As a result of these combined contributions of Bud6p and Kar9p, dynamic MT–cortex interactions are maintained as the bud grows and SPB separation begins. (3) During early spindle assembly, Bud6p associated capture and Kar9p-dependent delivery continue to focus MTs toward the bud. (4) As new astral MTs are organized by the respective SPB outer plaques (thick black lines) cortical capture begins at newly defined Bud6p sites at the bud neck. These interactions effectively restrict the new pole to the mother cell (irrespective of Kar9p initial presence on both SPBs; Fig. 4 C). (5) Progressively, Kar9p is recruited solely at the SPBd (spindle > 1 μm). (6) Coupled to continued SPB separation, MTs generated by the SPBm are no longer directed toward the bud and the spindle becomes aligned as it reaches final preanaphase length (∼2 μm).