FIGURE 1:

Nuclear movements in S. cerevisiae. Different modes of cMT plus-end capture and pulling on cMTs direct nuclei during the cell cycle of S. cerevisiae. G1 phase: cMTs arising from the SPB grow and shrink, but do so more slowly than cMTs in animal cells (Carminati and Stearns, 1997). Nuclei can be pushed when growing cMTs hit the cortex (Shaw et al., 1997). G1/S phase: actin cables emerge from the incipient bud site and the bud neck in small-budded cells and are used as transport tracks by the type V myosin Myo2 (Pruyne and Bretscher, 2000). cMT plus ends can be captured by Myo2 because Kar9 (APC tumor-suppressor homologue) connects the plus end–tracking Bim1 (EB1 homologue) and Myo2 indicated by the pink dot. Thus, the captured cMT is transported along actin cables to the bud neck, where shrinking of this cMT initiates pulling of the nucleus towards the bud neck, as necessary for nuclear positioning (Miller and Rose, 1998; Tirnauer et al., 1999; Adames and Cooper, 2000; Yin et al., 2000; Kusch et al., 2002; Hwang et al., 2003). Metaphase: after SPB duplication and separation, the old SPB stays close to the bud neck and accumulates Kar9 (Liakopoulos et al., 2003). Cytoplasmic MTs emanating from this SPB can be captured, as described earlier, at their plus end by Myo2 running at this cell cycle stage mainly along actin cables nucleated at the polarisome of medium- and larger-sized buds. Transport of captured cMTs along these cables secures the position of the nucleus and the correct orientation of the metaphase spindle. Anaphase: before the onset of anaphase, dynein accumulates at the bud-proximal SPB, but dynein assumes a symmetric distribution at both SPBs soon after the pulling of the anaphase spindle through the bud neck is initiated (Grava et al., 2006). Dynein is transported as an inactive complex with Pac1 (Lis1 homologue) at the plus ends of cMTs (not shown), searching the bud cortex and later also the mother cell cortex (Markus and Lee, 2011; Markus et al., 2011). When the immobile cortical Num1 captures dynein (light green dot), Pac1 is released, and the activated dynein, still bound to the cMT, starts pulling the anaphase spindle through the bud neck (Lammers and Markus, 2015). The pulled cMT glides along the bud cortex. Frequently, the elongating anaphase spindle is pulled back into the mother cell (Yeh et al., 1995; Adames and Cooper, 2000), most likely when dynein, captured and activated by Num1 at the mother cortex, exerts higher pulling forces on the mother cell SPB. This Dyn1-Num1–controlled oscillation of the anaphase pulling can repeat up to four times before the anaphase spindle is fully elongated.