The leading ends of ER tubules track with the plus ends of MTs. Demembranated sperm was added to a crude interphase X. laevis egg extract containing the hydrophobic dye DiIC18 (in red) and a GFP fusion of the plus end tracking protein EB-1 (EB-1–GFP; in green). The samples were imaged after 30-min incubation. Shown is the merged image of the two fluorescence channels. The yellow arrowheads point at EB-1 comets, marking the growing plus ends of MTs and the tip of extending ER tubules. The blue arrowhead indicates the fusion site of an ER tubule with another. Time-lapsed images were acquired by spinning-disk confocal microscopy at 2-s intervals for 1 min. The video is shown at four frames per second. Still images of this video were used for Fig. 3 A. Bar, 2 µm.
