Cover picture: Lipid protein nanotubes made of microtuble protein (made of tubulin protein subunits shown as red-blue-yellow-green objects) that is coated by a mixture of cationic and neutral lipid bilayer membrane (drawn with yellow tails and green (cationic) and white (neutral) spherical heads), which in turn is coated by tubulin protein rings or spirals. A top view of the nanotubes is shown on the right. Changes in the membrane-charge-density modify—in a stepwise fashion—the inner diameter of the microtubule lumen. The image was created by P. Allen, University of California, Santa Barbara, based on x-ray scattering study and transmission electron microscopy images described in the article by Raviv et al. on page 278.