Molecular architecture of the ciliary tip revealed by cryo-electron tomography.

Cilia are essential organelles that protrude from the cell body. Cilia are made of a microtubule-based structure called the axoneme. In most types of cilia, the ciliary tip is distinct from the rest of the cilium. Here, we used cryo-electron tomography and subtomogram averaging to obtain the structure of the ciliary tip of the ciliate Tetrahymena thermophila. We show the microtubules in the tip are highly cross-linked with each other and stabilised by luminal proteins, plugs and cap proteins at the plus ends. In the tip region, the central pair lacks the typical projections and twists significantly. By analysing cells lacking a ciliary tip-enriched protein CEP104/FAP256 by cryo-electron tomography and proteomics, we discovered candidates for the central pair cap complex and explain potential functions of CEP104/FAP256. These data provide new insights into the function of the ciliary tip and inform about the mechanisms of ciliary assembly and length regulation.

Long-range superior mirages.

Superior mirages of simple appearance are occasionally observed over distances exceeding 70 km. These mirages cannot be explained in terms of standard textbook models; rather, they are shown to arise from fairly complex atmospheres. Two observations of different types, observed at Resolute Bay, Canada, are presented. The first is the basic three-image mirage in which one inverted and one erect image float above the object. The second is a single-image mirage in which the object is elevated but undistorted. For each, the most suitable atmospheric model contains several distinct atmospheres, and the first one requires sloped atmospheric layers as well.