A schematic model for spindle anchoring to the mitotic SPB. (A) A model for spindle anchoring in wild-type cells. The minus end of the spindle microtubule is anchored properly to the SPB through binding between Msd1 and the γ-TuC. For this stable binding, interaction of Wdr8 with Msd1 is essential. At the SPB, inward forces generated by Pkl1/kinesin-14 within a ternary complex containing Msd1 and Wdr8 are antagonized by opposing outward forces generated by Cut7/kinesin-5 localized to the nearby overlapping microtubule zones. The Msd1–Wdr8–Pkl1 complex may play an additional, parallel role at the SPB by acting as a physical barrier resisting the Cut7/kinesin-5–mediated force. For simplicity, another separating SPB is not depicted. (B) If the nonmotile Pkl1 rigor mutant is tethered to the SPB, Pkl1^rigor is enough for the spindle-anchoring role as the physical barrier against Cut7/kinesin-5–mediated outward forces even in the absence of Msd1 or Wdr8. (C) Protrusion of the spindle microtubules in the absence of Pkl1 (also in the msd1Δ or wdr8Δ deletion). In the absence of the ternary complex, Cut7/kinesin-5–driven outward forces induce the protrusion of the minus end of the spindle microtubule beyond the SPB. Cut7 and occasionally the γ-TuC (Toya et al., 2007) are localized to this protruding microtubule minus end. NE, nuclear envelope.