S. Grava and P. Philippsen AgBik1 (aa 1-81) shares 70.4% identity with ScBik1. The Bik1-Cherry signal was observed throughout the length of hyphae, especially at cortical sites, adjacent to SPBs and at hyphal tips (Figure 1A). As suggested in budding yeast, the colocalization of Bik1 with SPBs could be either due to Bik1 binding to the kinetochore or to the end of very short cMTs (Lin et al., 2001). As shown in Figure 1B, AgBik1 also could be observed as bright dots at cMT ends. Those dots, in continuous motion and frequently moving along the cell cortex, clearly colocalized with the tip of GFP-labeled MTs (Figure 1, B and C). ends can be 25 m distant from ends and long cMTs can overtake several SPBs or nuclei to ﬁnally reach the hyphal tip (Figure 1C). A. gossypii cMTs Grow Faster and Shrink Less Frequently than Saccharomyces cerevisiae cMTs In budding yeast, cMT growth phases and shrinkage phases can both last for more than a minute with a catastrophe frequency of 2.16 events/min and a rescue frequency of 1.80 events/min (Adames et al., 2001). To measure MT dynamics in A. gossypii, we tracked 18 cMTs showing a Bik1 signal at their tip for 0.5–2.5 min. They were followed until the green and red ﬂuorescent signals at their ends became faint and fuzzy, suggesting that they were moving in an- other focal plane. Interestingly, those 18 cMTs were in con- stant growing phase and did not show any dynamic insta- bility. One cMT steadily grew 13.2 m in 168 s. The small variations in MT length observed in Figure 2A probably resulted from measurement mistakes due to slight move- ments of the SPB and cMTs out of the focal plane. Three consecutives frames of cMT shrinkage were never observed during the 213 time frames of cMT measurements. For those 18 cMTs, we calculated that the cMT polymerization rate was 0.108 0.035 m/s (mean SD; n 18; Figure 2A). Interestingly, this rate is 3 times higher than the rate of budding yeast (A. gossypii, 6.5 m/min vs. S. cerevisiae, 2.2 m/min; Tirnauer et al., 1999; Adames et al., 2001; Caudron 3682 Figure 1. Bik1 protein localizes at tips of growing cMTs. (A) 1Z plane picture has been taken in the middle of an 50- m-long hypha expressing Bik1-Cherry (red) GFP-Tub1(green). (B) White symbols show Bik1 dots at the tip of growing cMTs. The yellow arrowhead points out an apparently shrinking cMT. (C) A very long cMT reaches the hyphal tip. Asterisk (*) and arrowheads track the cMT tip and the SPB, respectively. Time is indicated in sec- onds. Bars, 5 m. et al., 2008). This increased cMT polymerization rate could partly explain why cMTs are overall longer in A. gossypii compared with budding yeast. In our whole GFP-Tub1 Bik1-Cherry movie collection ( 100 cMTs), we could only observe a very low number of shrinking cMTs (n 6). In contrast to budding yeast, shrink- ing cMTs in A. gossypii showed a very weak or no Bik1 signal at their tip (Figure 1B, yellow arrowhead). For those six cMTs, we calculated that the cMT depolymerization rate was 0.272 0.075 m/s (mean SD; Figure 2B). This shrinkage rate is also 3 times higher than in budding yeast (A. gossypii, 16.3 m/min vs. S. cerevisiae, 4.8 m/min; Adames et al., 2001). Interestingly, in addition to the bright spot visible at the cMT tip, lower intensity Bik1 spots were visible along the cMT (Figures 1B and 3A). We hypothesized that those addi- tional Bik1 spots can either be brought to the cMT tip by a end directed kinesin motor or moved toward the end via a end-directed motor, or alternatively be located at the tip of a short cMT bundled together with a longer cMT. Figure 2C shows that two Bik1 spots located at the tip of two cMTs emanating from different SPBs can eventually fuse (ﬂuores- cence intensity at t30: spot S1 10.8, spot S2 7.8; ﬂuorescence intensity at t36: spot S 17.6). This suggests that a tip dot of Bik1 could attach to another cMT and move along it to ﬁnally form parallel cMT bundles. Bundling of cMT from one SPB or different SPBs may be a way to increase cMT stability. Alto- gether, the very low frequency of cMT catastrophes, the higher cMT polymerization rate, and the possibility that cMTs form parallel bundles can at least partially explain why cMT grow longer in A. gossypii than in budding yeast. MTs Explore the Cytoplasm and Interact with the Cell Cortex to Induce Nuclear Movements Interactions between cell cortex and MTs can generate movements of the SPBs and nuclei. Here, we could monitor three different types of dynamic interactions between cMTs Molecular Biology of the Cell