Microtubule “Plus-End-Tracking Proteins” The End Is Just the Beginning

There are several key unanswered questions about the +TIPs. One is the mechanism of release of +TIPs from the trailing edge of polymerizing MTs. An important barrier to progress is the absence of an in vitro system that faithfully recapitulates both the binding and release that characterize plus end tracking. As +TIP partners/regulators may be important to reconstitute +TIP treadmilling in vitro, future progress may come from systems with additional purified components, or from ones based on crude extracts. Another question is the in vivo relationship between different +TIPs. It appears that CLIP-170 and EB1 can reside on the same growing MT end (Akhmanova et al., 2001xAkhmanova, A., Hoogenraad, C.C., Drabek, K., Stepanova, T., Dortland, B., Verkerk, T., Vermeulen, W., Burgering, B.M., De Zeeuw, C.I., Grosveld, F., and Galjart, N. Cell. 2001; 104: 923–935Abstract | Full Text | Full Text PDF | PubMed | Scopus (290)See all References)(Akhmanova et al., 2001). Is there cooperation or competition between these proteins, and could such interactions be a nodal point for regulating the repertoire of plus end behaviors?In addition, it is not known whether +TIPs can provide stable attachments. It will be important to determine if their association with MTs is stabilized (i.e., if they treadmill less) when MT plus ends interact with target sites. An alternative, suggested for CLIP-170 at the kinetochore, is that +TIPs mediate the initial attachment but then dissociate (Dujardin et al., 1998xDujardin, D., Wacker, U.I., Moreau, A., Schroer, T.A., Rickard, J.E., and De Mey, J.R. J. Cell Biol. 1998; 141: 849–862Crossref | Scopus (116)See all References)(Dujardin et al., 1998). Real-time methods where the turnover of +TIPs at the MT end can be measured may help distinguish between these possibilities. Although budding yeast is not famous for the awesome power of its cytology, the ability to see single MTs interacting with target sites on the membrane provides unique spatial resolution that should facilitate these experiments. Finally, do the +TIPs only function at the MT plus end? Several +TIPs interact with proteins that associate with or regulate the behavior of the MT minus ends (Chen et al. 1998xChen, X.P., Yin, H., and Huffaker, T.C. J. Cell Biol. 1998; 141: 1169–1179Crossref | Scopus (71)See all References, Chen et al. 2000xChen, C.R., Chen, J., and Chang, E.C. Mol. Biol. Cell. 2000; 11: 4067–4077CrossrefSee all References). This raises the possibility that +TIPs might have an additional role in MT nucleation or in anchoring MT minus ends to the centrosome.Like motors and the proteins involved in microtubule nucleation, the CLIP-170 family and EB1 family proteins are highly conserved. We speculate that these proteins evolved because of the need for devices that distinguish the plus end from the body of the MT. Although the plus end has a unique shape, its large size (>25 nm in diameter) makes it an unwieldy object to recognize at the molecular level. +TIPs may solve this problem; if copolymerized with tubulin, +TIPs may “tag” MT plus ends. By interacting with different partners, +TIPs could serve as molecular adaptors, providing links to a large repertoire of signals and target sites. In the cell, this diversity of plus end interactions may be fundamental to the regional control of MT dynamics, MT attachment, and the assembly of complex MT-based structures necessary for cell division and morphogenesis.