Green fluorescent protein
Aequorea victoria (jellyfish)
Fluorescence is an optical phenomenon of certain chemical compounds. These compounds absorb higher energy light and re-emit lower energy light. They can be used for instance to visualize ultraviolet or x-ray radiation. Fluorescent marker molecules are a common tool in several areas of biochemistry. Jellyfish are unusual creatures of the sea and these organisms produce a protein that can fluoresce, the Green Fluorescent Protein (GFP). When irradiated with short-wavelength light, the protein first absorbs this light and then emits light of a somewhat longer wavelength. The emitted light's color is green. In molecular biology GFP has become a popular tool for visualization. GFP can for instance be co-expressed with other proteins as a fusion product. This not only can help such proteins fold more easily, but also makes it easy to see them by the green light that GFP emits. Many proteins have chromophores, substances that are closely associated with the protein and that can absorb and re-emit light. These are usually small molecules, often hydrophobic. GFP is peculiar, because the green light is not produced by an independent chromophore, but is made from part of the amino acid sequence.
Part of its sequence, serine-tyrosine- glycine, through a chemical reaction, a post-translational modification, is changed and becomes a chromophore that is part of the protein itself. The structure of GFP is also very interesting. The shape of the protein is an 11-stranded beta barrel with a long helix running through the center of the barrel. In the middle of this helix sits the chromophore. Compare this for instance with a protein like retinol-binding protein (see PDB:1BRP). That protein also has a barrel shape, formed by 8 beta strands, with the retinol bound in the center of the barrel. Other similar proteins have similar geometry, but to have a chromophore attached to a helix running through the center of the barrel is unusual. Quite a fitting molecule for an unusual organism like jellyfish.
Protein Data Bank (PDB)
Ormo, M. Cubitt, A.B. Kallio, K. Gross, L.A. Tsien, R.Y. Remington, S.J.; "Crystal structure of the Aequorea victoria green fluorescent protein."; Science; (1996) 273:1392-1395 PubMed:8703075.
author: Arno Paehler