Coordinating mitosis with cell polarity: Molecular motors at the cell cortex

Abstract

In many cell divisions, the position of the spindle apparatus is coordinated with polarity signals at the cell cortex so that copies of the genome are delivered to regions of the cell that are designated for differential inheritance by the two progeny. To coordinate spindle position with cell polarity, the spindle interfaces with elements on the cortex, where molecular motors often produce the forces that power displacement. Here we describe the molecular pathways by which cortical motors translocate the spindle in budding yeast, where the mechanisms are understood relatively well, and we compare these pathways to spindle positioning processes in metazoan systems, where the molecular details are less well understood.

Figure 1

Models for spindle displacement by cortical motors. (A) Sequence of spindle positioning events during cell division in budding yeast. First, the plus ends of one set of microtubules are delivered to the bud. Second, microtubules that are attached to the bud neck shorten, drawing the spindle and nucleus toward the bud. Finally, microtubules are pulled along the bud cortex, toward the distal bud tip. This pulls the spindle and nucleus into the bud neck, and positions the anaphase spindle across the nascent site of cytokinesis. (B) Myosin-V transports microtubule plus ends along actin cables toward the bud. Arrow indicates the direction of myosin-V motility, determined by the polarity of actin cables that are nucleated at the bud tip and bud neck. Arrowhead indicates the force applied to the microtubule. (C) Capture-shrinkage at the bud neck. Microtubule plus ends are captured by the forming-interacting protein, Bud6, and are then induced to shorten by a mechanism that is likely to involve the depolymerase activity of kinesin-14. (D) Microtubules are pulled along the bud cortex by the minus-end directed motor dynein and its activator, dynactin. The dynein-dynactin complex is anchored to the cortex by Num1.