Fig. 2. Interphase microtubule arrays are composed of overlapping half bundles of 2-3 autonomously behaving microtubules. (A) Shown here are sequential images from a time-lapse movie of a cell expressing GFP:-tubulin (YY105). Images were taken at approximately 2-second intervals. Arrows and arrowheads mark the plus ends of individual microtubules, highlighting the presence of multiple microtubules in each bundle. The arrow points to a microtubule that is shrinking and the arrowheads to microtubules that are growing. The asterisk marks the site where antiparallel microtubules overlap. Bar, 2 µm. (B) The same time-lapse movie presented as a kymograph with time in seconds along the X-axis and length of cell along the Y-axis. The time points shown in A are delineated with a rectangle on the time line of the kymograph. The region of the microtubule overlap that corresponds to the still images is marked with an asterisk; this also marks the region where the SPB is found. The lines descending from the minus-end overlap zone towards the cell tip reflect growth of a single microtubule (white arrow). Lines ascending from the cell tip toward the minus-end overlap zone reflect depolymerization (black arrow). The triangular regions of increased fluorescence intensity where growth and shrinkage lines intersect is due to the presence of one growing and one shrinking microtubule. Bar, 2 µm. (C) Comparison of pixel intensity along the microtubule bundle at single time points. The graph on the left compares the pixel intensity plots of two consecutive time points (seen in Fig. 2A), 154 and 156. The plots are almost identical, showing that there is little change in the composition of the microtubule bundle and that the two intensity plots show similar but not identical patterns between the two time points. The plots show five major changes in fluorescence intensity across the bundle. The half bundle on the left (Fig. 2A, time 154) shows three intensity peaks on the graph, which are marked by arrows and arrow heads in Fig. 2A (time 154). The overlap zone shows the area of greatest intensity. The half bundle on the right in Fig. 2A (time 154) shows a more dramatic decrease in intensity from the overlap zone and a pixel intensity of two microtubules. This is also evident in the graph shown here. The graph on the right compares time 154 with time 179, showing there were major changes in the intensity distribution and therefore the number of microtubules along the length of the bundle over time (see Movie 1, http://jcs.biologists.org/supplemental/).