Microtubule Plus‐End‐Tracking Proteins

Abstract

Microtubules are dynamic cytoskeletal components necessary for several intracellular processes, including cell division, differentiation, migration and intracellular transport. Microtubule plus‐end‐tracking proteins (+TIPs) consist of a highly diversified group of evolutionary conserved families of proteins that preferentially accumulate at the plus‐ends of microtubules. Importantly, +TIPs are capable of interacting with each other, which allows the establishment of complex protein networks greatly implicated in the regulation of microtubule behaviour. These networks play an important role mediating the interactions between microtubules and several cellular structures, such as membranes, kinetochores and the actin cytoskeleton, thereby influencing cellular architecture and coordinating diverse biological processes. For these reasons, +TIPs have been extensively studied, including the peculiar structural features allowing for the generation of networks, the mechanisms behind microtubule plus‐end‐tracking and their respective roles in the cell.

Key Concepts

  • +TIPs consist of an evolutionarily conserved, yet highly diversified group of proteins that preferentially accumulate at the plus‐end of microtubules regulating their dynamics.
  • In their structure +TIPs can comprise specialized domains fit to directly interact with tubulin, or able to bind proteins capable of autonomously associating with tubulin.
  • The establishment of +TIPs networks responsible for the regulation of microtubule dynamics is important for numerous biological processes in interphase and mitosis.

Keywords: microtubule; microtubule dynamics; microtubule‐associated proteins; mitosis; plus‐end‐tracking; cytoskeleton

Figure 1. +TIP distribution in interphase mammalian cells. (a) Schematic drawing of a cell with distinct MT populations emerging from the microtubule organising centre (MTOC). The growing MTs (green) hold +TIPs (purple) at their end that promote MT polymerisation, whereas MTs undergoing catastrophe (red) comprise a different +TIP composition. (b and c) Immunofluorescence of fixed HeLa cells stably expressing EB1‐EGFP (in b) or EGFP‐CLASP1 (in c) (green) and stained for α‐tubulin (red in b), EB1 (red in c) and counterstained with 4,6‐diamino‐2‐phenylindole (DAPI) to reveal the DNA (blue). Higher magnifications of selected region‐of‐interest (ROI) shows the distribution of these +TIPs at the distal ends of MTs. (d and e) Dynamic behaviour of +TIPs in the intracellular space at the distal ends of microtubules. (d) Colour‐coded time projection of a sequence of 15 frames, corresponding to 30 s. A red‐green‐blue (RGB) gradient filter was applied to each frame, consequently leading to early frames being red‐coloured, whereas latter frames are blue labelled. As a result, moving features (microtubule tips) are seen as coloured tracks. Scale: 10 µm. (e) Kymograph (plot of distance vs time) of an ROI of live‐cell imaging of a HeLa cell transfected with EB1‐EGFP. Each row is obtained by maximum‐intensity projection along the small axis of the selected ROI. Vertical tracks represent static features and diagonal tracks are observed for moving particles. Horizontal scale: 10 µm; Vertical: 10 s.
Figure 2. Mechanisms of +TIPs delivery to microtubule ends. Schematic illustration picturing the various mechanisms used by +TIPs to locate to plus‐ends. Plus‐end‐tracking proteins can reach MT ends by diffusion from the cytoplasm or along the MT lattice. Autonomous +TIPs, such as EBs, can recognise and directly bind to MT plus‐ends through their calponin homology (CH) domain. They are also capable of carrying other +TIPs to that region through hitchhiking. Particularly, +TIPs with an SxIP domain can bind to the end‐binding homology region (EBH), whereas some CAP‐Gly proteins identify and attach to composite binding sites of EBs C‐terminus (EEY). Alternatively, +TIPs can also be brought to the plus‐ends through a kinesin‐mediated transport. Some +TIPs could also be able to co‐polymerise with soluble tubulin dimers and then move to the MT plus‐end.
Figure 3. Localization of CLASP1 in mitosis. Mitotic HeLa cells stably expressing EGFP‐CLASP1 (green) were fixed, stained for α‐tubulin (red) and DNA was counterstained with DAPi (blue). EGFP‐CLASP1 can be found at the spindle, centrosomes, midzone and midbody, as indicated by arrows in the second and third panel, respectively. Higher magnification on the first image of selected ROI show EGFP‐CLASP1 at kinetochores bound to MTs (k‐fibres).
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Further Reading

Gouveia SM and Akhmanova A (2010) Cell and molecular biology of microtubule plus end tracking proteins: end binding proteins and their partners. International Review of Cell and Molecular Biology 309: 59–140.

Gupta KK, Alberico EO, Nathke IS and Goodson HV (2014) Promoting microtubule assembly: a hypothesis for the functional significance of the +TIP network. BioEssays: News and Reviews in Molecular, Cellular and Developmental Biology 36 (9): 818–826.

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Stehbens SJ, Paszek M, Pemble H, et al. (2014) CLASPs link focal‐adhesion‐associated microtubule capture to localized exocytosis and adhesion site turnover. Nature Cell Biology 16 (6): 561–573.

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Pereira, Ana L, and Maiato, Helder(Apr 2015) Microtubule Plus‐End‐Tracking Proteins. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0025979]