Bos taurus (Bovine)
Guanosine nucleotides are a popular signaling molecule. One class of proteins that assists in using them for signaling are the G-proteins. Many cellular receptors are coupled withe such G-proteins. Collectively they are called G-protein coupled receptors (GPCR). The receptors respond to various external events by activating their associated G-proteins. GPCR, as varied as they may, share as a common feature that they contain a membrane-bound domain with 7 alpha-helices. Long before Deisenhofer, Huber and Michel determined the first membrane protein structure in the early 1980s, Henderson and Unwin had studied bacteriorhodopsin by electron microscopy. Not too long ago the first X-ray structure of bacteriorhodopsin was determined, providing a more detailed picture. GPCR, since they are involved in signaling events, are of great interest to the pharmaceutical industry which hopes to design cures for diseases by interfering with signaling pathways. As interesting as a bacterial rhodopsin may be, they would prefer something more human.
While the structure shown is not quite human yet, it comes closer than its bacterial cousin. It shows bovine rhodopsin involved in the vision process, light being the stimulus. As expected, it has a 7-helix structure. Two sugar molecules can also be seen bound to asparagine sidechains. They are N-acetyl glucosamine sugars. This is a common feature of many proteins, in particular those that have receptor functions. Such a protein is called glycosylated. Somewhat further down is a retinal molecule buried inside the receptor. The retinal, when exposed to light, changes its chemical conformation and this change informs the body of the event.
Protein Data Bank (PDB)
Palczewski, K. Kumasaka, T. Hori, T. Behnke, C.A. Motoshima, H. Fox, B.A. Le Trong, I. Teller, D.C. Okada, T. Stenkamp, R.E. Yamamoto, M. Miyano, M.; "Crystal structure of rhodopsin: A G protein-coupled receptor."; Science; (2000) 289:739-745 PubMed:10926528.
author: Arno Paehler