binding. Furthermore, as acetylation occurs only after microtubule polymerization, the enzyme responsible for this modification should probably localize to the microtubule lumen. The preference of ER sliding for acetylated microtubules may indicate a functional difference between this type of ER motility and that mediated by the tip attachment complex. Intermembrane contacts are important for many functions of the ER and other membranous compartments [9]. The authors’ results led them to hypothesize that the sliding mechanism may be used as a way for ER tubules to find and contact other organelles along a subpopulation of microtubules. To explore this possibility, Friedman et al. [8] tracked the movements of two other organelles, mitochondria and endosomes, with respect to both ER and acetylated microtubules. A majority of both organelles remained in persistent contact with the ER. However, only mitochondria appeared to also localize preferentially to acetylated microtubules, suggesting that ER contacts with mitochondria, but not endosomes, are enriched along acetylated microtubules. Consistent with this finding is the fact that both ER and mitochondria are cargoes of kinesin-1, while endosomes are moved by KIF16B, a member of the kinesin-3 family [15]. Further, work by Cai et al. [6] showed that another kinesin-3 family member, KIF1A, displayed no preference for acetylated microtubules in COS-7 cells. By biasing protein association with a particular subset of microtubules, acetylation allows for subpopulations of microtubules to act as compartments along which specific cargoes can find each other, and be found in return. In a sense, these microtubule compartments can be thought of as cellular pubs that attract a specific cargo crowd for mingling. Microtubule acetylation is also probably involved in the regulation of cell migration both in fibroblasts [16] and neurons [17]. It is attractive to speculate that polarization of moving cells requires kinesin-dependent recruitment of selective cargoes to the leading edge of migrating cells. Additional investigation is necessary to determine whether this selective motor recruitment applies to other microtubule modifications and other motor proteins. It is possible that at least some other microtubule motors have a higher affinity for either a specific tubulin isoform or a particular post-translational modification, similar to the preferential recruitment of kinesin-1 to acetylated microtubules. Such selectivity could create multiple microtubule compartments to recruit particular cargoes — different pubs for different crowds (Figure 1). Once bound to microtubules, the cargo can undergo bidirectional transport to facilitate its movement through the crowded cytoplasm. Cargoes recruited to the same subset of microtubules will interact with much higher efficiency, thus promoting exchange of molecules between particular cell compartments. If this simple model is correct, it indicates that, in addition to their role as tracks for long-distance transport, microtubules serve an important role as scaffolds for the organization and compartmentalization of the otherwise randomly distributed cellular components. References 1. Westermann, S., and Weber, K. (2003). Post-translational modifications regulate microtubule function. Nat. Rev. Mol. Cell Biol. 4, 938–947. 2. Liao, G., and Gundersen, G.G. (1998). Kinesin is a candidate for cross-bridging microtubules and intermediate filaments. Selective binding of kinesin to detyrosinated tubulin and vimentin. J. Biol. Chem. 273, 9797–9803. 3. Reed, N.A., Cai, D., Blasius, T.L., Jih, G.T., Meyhofer, E., Gaertig, J., and Verhey, K.J. (2006). Microtubule acetylation promotes kinesin-1 binding and transport. Curr. Biol. 16, 2166–2172. 4. Dompierre, J.P., Godin, J.D., Charrin, B.C., Cordelieres, F.P., King, S.J., Humbert, S., and Saudou, F. (2007). Histone deacetylase 6 inhibition compensates for the transport deficit in Huntington’s disease by increasing tubulin acetylation. J. Neurosci. 27, 3571–3583. 5. Konishi, Y., and Setou, M. (2009). Tubulin tyrosination navigates the kinesin-1 motor domain to axons. Nat. Neurosci. 12, 559–567. 6. Cai, D., McEwen, D.P., Martens, J.R., Meyhofer, E., and Verhey, K.J. (2009). Single molecule imaging reveals differences in microtubule track selection between Kinesin motors. PLoS Biol. 7, e1000216. 7. Hammond, J.W., Huang, C.F., Kaech, S., Jacobson, C., Banker, G., and Verhey, K.J. (2010). Posttranslational modifications of tubulin and the polarized transport of kinesin-1 in neurons. Mol. Biol. Cell 21, 572–583. 8. Friedman, J.R., Webster, B.M., Mastronarde, D.N., Verhey, K.J., and Voeltz, G.K. (2010). ER sliding dynamics and ER-mitochondrial contacts occur on acetylated microtubules. J. Cell Biol. 190, 363–375. 9. Voeltz, G.K., Rolls, M.M., and Rapoport, T.A. (2002). Structural organization of the endoplasmic reticulum. EMBO Rep. 3, 944–950. 10. Terasaki, M., Chen, L.B., and Fujiwara, K. (1986). Microtubules and the endoplasmic reticulum are highly interdependent structures. J. Cell Biol. 103, 1557–1568. 11. Waterman-Storer, C.M., and Salmon, E.D. (1998). Endoplasmic reticulum membrane tubules are distributed by microtubules in living cells using three distinct mechanisms. Curr. Biol. 8, 798–806. 12. Allan, V., and Vale, R. (1994). Movement of membrane tubules along microtubules in vitro: evidence for specialised sites of motor attachment. J. Cell Sci. 107, 1885–1897. 13. Wozniak, M.J., Bola, B., Brownhill, K., Yang, Y.C., Levakova, V., and Allan, V.J. (2009). Role of kinesin-1 and cytoplasmic dynein in endoplasmic reticulum movement in VERO cells. J. Cell Sci. 122, 1979–1989. 14. Nogales, E., Whittaker, M., Milligan, R.A., and Downing, K.H. (1999). High-resolution model of the microtubule. Cell 96, 79–88. 15. Hoepfner, S., Severin, F., Cabezas, A., Habermann, B., Runge, A., Gillooly, D., Stenmark, H., and Zerial, M. (2005). Modulation of receptor recycling and degradation by the endosomal kinesin KIF16B. Cell 121, 437–450. 16. Gundersen, G.G., and Bulinski, J.C. (1988). Selective stabilization of microtubules oriented toward the direction of cell migration. Proc. Natl. Acad. Sci. USA 85, 5946–5950. 17. Creppe, C., Malinouskaya, L., Volvert, M.L., Gillard, M., Close, P., Malaise, O., Laguesse, S., Cornez, I., Rahmouni, S., Ormenese, S., et al. (2009). Elongator controls the migration and differentiation of cortical neurons through acetylation of alpha-tubulin. Cell 136, 551–564. Department of Cell and Molecular Biology, Northwestern University Feinberg School of Medicine, Chicago, IL 60611, USA. *E-mail: vgelfand@northwestern.edu DOI: 10.1016/j.cub.2010.08.058 Insect Vision: A Few Tricks to Regulate Flight Altitude A recent study sheds new light on the visual cues used by Drosophila to regulate flight altitude. The striking similarity with previously identified steering mechanisms provides a coherent basis for novel models of vision-based flight control in insects and robots. Dario Floreano and Jean-Christophe Zufferey Insects predominantly use vision to steer, to regulate their flight speed and height, to avoid impending obstacles, to chase moving targets, and to land on objects. Their evolutionary success, ability to fly in complex environments, and the Dispatch R847