Green fluorescent protein (GFP) serves as a marker for protein localization as well as monitoring protein- protein interactions (Tsien, 1998). GFP has a beta-barrel tertiary structure consisting of eleven beta-strands with alpha helix running through the center (Deponte, 2012). The formation of chromophore requires molecular oxygen as well as four conserved residues, which are Y66, G67, R96 and E222 (Deponte, 2012). When a photon from ultraviolet light strikes the chromophore thus exciting it to release a photon of green light to lower the energy in the system and bring it back to its original state (Stepanenko et al., 2008).
Some studies showed that when GFP fused to the N-terminus of the protein, it had no effect on the function or the property of the tagged protein (Tsien, 1998). However, if the GFP was tagged to the C-terminus, it had an effect on the solubility of the tagged protein (Tsien, 1998). Furthermore, gfp, a gene that encodes GFP, can be added either to the N- or C- terminus as well as somewhere within the sequence of the gene encoding protein of interest (Deponte, 2012). This can be done by the use of restriction enzymes that would cut both the gfp and the gene encoding protein of interest to ensure proper ligation between the two genes (Deponte, 2012). GFP can fluoresce different colors including green, yellow and red at approximately 485nm, 540nm, and 570nm respectively, due to a mutation that results in substitutions of amino acid residues