Figure 9.

Motors in anaphase in U. maydis. In anaphase A, slow spindle elongation is supported by internal forces that might be generated by kinesin-5 localized at the mid-zone. Kinesin-5 also accumulates at the cell poles. Astral MTs in anaphase A are short and no dynein is located at their ends, except during rare rotational motion. When the spindle reaches a length of 1.5–2 μm, long astral MTs appear that carry dynein at their plus-end. Interestingly, more dynein is found at the tip of MTs that extend into the mother, which nicely corresponds to the faster velocity of spindle halves moving into the mother after laser dissection. Both spindle halves occasionally undergo bidirectional motility, which indicates that counteracting forces are applied on each individual spindle pole (blue arrows). The appearance of long astral MTs coincides with a four-fold increase in spindle elongation rate in early anaphase B. Dynein mutants fail to establish long distal MTs and the elongation rate does not increase, which suggests that dynein pulls on the spindle poles. In addition, kinesin-5 localizes at the mid-zone in anaphase B and seems to support spindle elongation up to 7–8 μm in dynein mutants, which indicates that it participates in spindle elongation. Kinesin-5 then disappears from the mid-zone, while dynein still pulls on the poles in late anaphase B. Note that additional forces might participate in anaphase A (indicated by ?).