The k-fiber lengthening under sustained force occurs by suppressing depolymerization at plus- and minus-ends, related to Fig. 3. Microneedle manipulation of individual k-fiber photomarked with PA-GFP-tubulin (white) in metaphase PtK1 cell reveals the mechanism of k-fiber lengthening under force. The microneedle (Alexa 555, yellow) exerts force on the k-fiber (SiR-tubulin, magenta) for minutes, and the photomark remains a constant distance from the pole but at a persistently increasing distance from the plus-end as the k-fiber lengthens, indicating a suppression of depolymerization at both ends. Scale bar, 4 µm. Video was collected using a spinning disk confocal microscope at one frame every 10 s. Video has been adjusted to play back at a constant rate of 15 frames/s. Video corresponds to still images from Fig. 3 B.
