Recent work from different laboratories has shown that a family of proteins is recruited to the tips of microtubules (reviewed in Akhmanova and Steinmetz, 2008). In particular, most of these proteins are recruited specifically to microtubule tips undergoing polymerisation. Examples of cells transfected with some of these microtubule tip-proteins (conjugated with EGFP) are shown in the following four videos:
A B16 melanoma cell expressing GFP-CLIP-170.
A goldfish fibroblast expressing GFP-EB1, that was injected with Cy-3 tubulin.
A goldfish fibroblast expressing GFP-EB3.
An MRC5 cell transfected with RFP-tubulin and EB3-GFP showing the localization of EB3 at growing microtubule tips. Video was kindly provided by Ilya Grigoriev and Anna Akhmanova (Rotterdam)
Dual label microscopy shows that microtubule tip complexes track into focal adhesions:
Video of a fish fibroblast expressing GFP-CLIP-170 and DsRed-zyxin recorded on a laser scanning confocal microscope. Note the repeated penetration of microtubule tips into the peripheral focal adhesions. (From Krylyshkina et al., 2003)
By using evanescent wave microscopy it can be demonstrated that the tracks are very close to the ventral cell surface (Krylyshkina et al., 2003). These tracks bring microtubule tip complexes close enough to focal adhesions for molecular exchanges to occur:
Evanescent wave microscopy of a fish fibroblast transfected with GFP-CLIP-170 and DsRed zyxin. The microtubule tips enter the layer of exciting radiation within 100 — 200nm above the substrate and track close to the ventral surface into focal adhesions. (From Krylyshkina et al., 2003)