However, as well as increasing catastrophe frequency, Klp5/6 in vivo is also reported to increase the rescue frequency and growth rate. Overall, the effects of kinesin-8 on interphase microtubules in budding yeast, fission yeast and human cells suggest that different members of the kinesins-8 family can increase not only the catastrophe frequency but also the rescue frequency, growth rate and duration of pauses of dynamic microtubules in vivo. Furthermore, these phenomena can arise through either direct or indirect effects of the kinesins-8. Resolving the mechanisms responsible for the diverse in vivo properties reported for the kinesins-8 requires in vitro reconstitution experiments to distinguish direct from indirect effects. Perhaps influenced by Kip3 work, most reconstitution experiments so far have focused on depolymerisation of brain microtubules stabilized with taxol or GMPCPP. Kip3 depolymerizes GMPCPP brain microtubules but any effect on dynamic microtubules remains to be demonstrated in vitro. For Kif18a the ability to depolymerise GMPCPP stabilised microtubules in vitro is itself controversial. Kif18a also fails to depolymerise Taxol stabilised microtubules, although in the presence of a nonhydrolysable ATP analogue it does sequester tubulin into ring structures similar to those formed by MCAK, suggesting it shares some of MCAKs functional as well as structural features. In assays on dynamic microtubules Kif18a has no effect on the stability of existing microtubules, but does block polymerisation of microtubules by Evofosfamide acting as a capping protein. In vitro experiments on full-length Klp5 and 6, expressed as a complex in baculovirus, revealed plus-end-directed sliding of brain microtubules at 39 nm s21, whilst Klp6 motor domain alone drives sliding at 56 nm s21. Klp5/6 is reported to have no in vitro depolymerase activity, either on GMPCPP or Taxol stabilised brain microtubules or on shrinking GDP brain microtubules. Since the biochemical behaviour of kinesin can be different for microtubules from different species we have examined the effect of Klp5/6 on dynamic S. pombe microtubules, both in vitro and in vivo. Our data suggest a new working model for Klp5/6 in vivo. We propose Klp5/6 tubulin complexes initially promote the birth of new microtubules, and thereafter continuously land on the growing microtubule tip, attempting to keep pace with the tip as it grows, but only succeeding at cell ends, where the microtubule tips lodge and their growth slows down in compression. Only then, within the context of the cell end, does Klp5/6 promote microtubule catastrophe. This mechanism, which allows Klp5/6 both to promote nucleation and to amplify an intrinsic tendency for microtubule tips to catastrophise under compression, may have relevance to other kinesins-8. Klp6440His was gel filtered and an aliquot taken from the peak of monodisperse protein. This repurified protein produced both a dose-dependent increase in the number of microtubules formed from S. pombe tubulin and a dosedependent reduction in mean microtubule length, consistent with it promoting microtubule nucleation. We considered two possible classes of nucleation mechanism. First, Klp5 and Klp6 binding to tubulin heterodimers might alter their conformation to favour assembly. Second, dimeric Klp5 and Klp6 might link tubulin heterodimers together to stabilise nascent microtubule nuclei.