A fusion protein is a protein consisting of at least two domains that are encoded by separate genes that have been joined so that they are transcribed and translated as a single unit, producing a single polypeptide. Virtually almost all recombinant proteins are now prepared using fusion domains also known as "tags" (See the full list of recombinant protein tags). Then the fusion protein is also called fusion tag protein or chimeric protein.
There are essentially two types of fusion proteins: the first of which consists of two proteins or protein subunits fused end-to-end and usually linked by a linker, and the second, in which amino acids from both donors are interspersed in the fusion protein product.
The use of tags helps to solve some serious problems:
• to simplify procedures of protein isolation
• to increase expression and solubility of the desired protein
• to simplify protein refolding and increase its efficiency
• to prevent proteolysis
Three of the most important uses of fusion proteins are:
1. as aids in the purification of cloned genes
2. as reporters of expression level
3. as histochemical tags to enable visualization of the location of proteins in a cell, tissue, or organism
A protein is fused to a protein can be easily and conveniently purified by affinity chromatography in purification such as staphylococcus protein A, glutathione-S-transferase (gst), maltose-binding protein (mbp) and cellulose-binding protein. The recombinant fusion proteins are used most often as fusion partners for reporter constructs are β-galactosidase, luciferase, and green fluorescent protein (GFP).
Among the fusion proteins, one of the most category is called fluorescent proteins such as green fluorescent protein (GFP), orange fluorescent protein (OFP) and yellow fluorescent protein (YFP). Fluorescent proteins such as green fluorescent protein (GFP) have enabled direct observation of dynamic intracellular processes.
Green fluorescent protein (GFP) is a fluorescent protein that was originally isolated from the luminous organ of the jellyfish Aequorea victoria. Unlike luciferase, GFP has an advantage that it does not require any substrate, luciferin, as well as ATP, O2, or Mg2+. GFP emits green light when excited by blue or UV light, and in many cases can be used on live, intact cells and organisms, thus ensuring the function of GFP as an autofluorescent protein (See more about GFP).
|GFP Molecule 3D Structure|
|GFP Molecule SDS-PAGE and purified protein|
Among fusion tags there are both short sequences (such as PolyHis, PolyArg, FLAG, c-Myc, Streptag, etc.) and large proteins (GST, MBP, etc.). In many cases the short sequences do not influence the tertiary structure of the molecule and its biological features while large fusion molecules are more frequently used for enhancement of solubility of desired proteins. Unlike the tags of short sequence, it is necessary to remove the large fusion tags from the recombinant construct.
There are many fusion tags containing both well proven tags and recently developed ones of various characteristics and different advantages and disadvantages. (See more about the details of fusion tags)
Pina AS, et al. (2014) Affinity tags in protein purification and peptide enrichment: An overview. Methods in molecular biology (Clifton, N.J.) 1129: 147-168.
Kosobokova EN, et al. (2016) Overview of fusion tags for recombinant proteins. Biochemistry (Mosc) 81(3): 187-200.