Photoreceptors are specialized cells that elaborate a cilium which becomes the inner and outer segments of the photoreceptor that house photopigments (a beautiful TEM image of a photoreceptor cilium and its inner/outer segment can be seen here)…. Cilium are specialized structures that come in two flavors, motile and non-motile. Motile cilium are found in places like the fallopian tubes or in the trachea to are designed to move in coordinated fashion. Mammalian organisms have a non-motile cilium in every cell. However, in some cells, the cilium become highly modified to become a structural basis for a sensory organ such as photoreceptors or hair cells in the ear.
A variety of diseases or functional deficits can be attributed to cilium related defects termed ciliopathies. This paper in Nature Neuroscience by Houbin Zhang, Ryan Constantine, Sergey Vorobiev, Yang Chen, Jayaraman Seetharaman, Yuanpeng Janet Huang, Rong Xiao, Gaetano T Montelione, Cecilia D Gerstner, M Wayne Davis, George Inana, Frank G Whitby, Erik M Jorgensen, Christopher P Hill, Liang Tong & Wolfgang Baehr addresses a common mechanism behind ciliary transport defects that impair signal transduction, ciliogenesis and maintenance of the cilium. It turns out that UNC119 is a lipid-binding protein that interacts with acylated N terminals of G protein α-subunits. The fundamental finding of this paper are a spectacular crystal structure of UNC119 revealing an immunoglobulin-like β-sandwich fold that forms a hydrophobic cavity capable of accommodating lipid moieties. Importantly, UNC119 deletion in both mouse and worm causes G protein trafficking deficiencies from mice to C. elegans.