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GFP (green fluorescent protein) is a protein from the jellyfish Aequorea victoria that fluoresces green by itself. The wild-type protein is 238 amino acids long and about 26.9 kDa, folds into a beta-barrel, and builds its own light-emitting chromophore from three of its residues using only molecular oxygen. Researchers fuse its gene to other genes so a protein or a promoter can be followed in living cells.
Definition and scope
The UniProt Swiss-Prot entry P42212 lists the wild-type protein as “Green fluorescent protein” from Aequorea victoria, with a single chain of 238 amino acids and a calculated mass of 26,886 Da. In the jellyfish its annotated role is as an energy-transfer acceptor: it converts the blue chemiluminescence of the photoprotein aequorin into green light. In the laboratory the same protein, and engineered variants of it, serve as a genetically encoded label.
“GFP” is also used loosely for the whole family of engineered green variants. EGFP (enhanced GFP), catalogued in FPbase, is the one most researchers mean when they say GFP today, and it is described separately below.
How GFP makes light
The structure deposited as RCSB PDB entry 1EMA (Crystal structure of the Aequorea victoria green fluorescent protein, Science 273:1392-1395, 1996) is classed with the keywords beta-barrel, autocatalytic and fluorophore. The protein is a barrel of strands wrapped around the chromophore, which sits inside it.
UniProt describes how the chromophore forms. It arises by autocatalytic backbone condensation between Ser-65 and Gly-67 together with oxidation of Tyr-66 to didehydrotyrosine, and “maturation of the chromophore requires nothing other than molecular oxygen”. No enzyme, cofactor or added substrate is annotated. That is why the gene alone is enough to make a cell glow, and why the protein works in many host organisms.
Size of GFP protein
Wild-type GFP is 238 amino acids and 26,886 Da according to UniProt P42212. The 1EMA crystal structure is an engineered construct with a Thr substitution at position 65, deposited with 236 residues and a molecular weight of 27.23 in the RCSB record, so “about 27 kDa” is the right order of magnitude for either.
The practical consequence is simple arithmetic: a GFP fusion adds roughly 27 kDa to the protein it is attached to. That is large compared with a short peptide tag, and it is the first number to check when a fused protein is small, or when a fusion has to be separated from its untagged partner on a gel by size.
Wild-type GFP and EGFP compared
| Property | Wild-type GFP (UniProt P42212) | EGFP (FPbase) |
|---|---|---|
| Excitation | Major absorbance peak at 395 nm, smaller peak at 470 nm | Maximum at 488 nm |
| Emission | Peak at 507-510 nm, shoulder at 545 nm | Maximum at 507 nm |
| Extinction coefficient | Not listed in the entry | 55,900 M-1 cm-1 |
| Quantum yield | Not listed in the entry | 0.6 |
| Brightness (FPbase) | Not listed in the entry | 33.54 |
| Oligomeric state | Annotated as a monomer | Weak dimer |
FPbase lists EGFP’s parent protein as avGFP, the Aequorea victoria protein. UniProt also notes that the exact emission maximum depends on the environment of the chromophore, and that mutants have been built with shifted spectra, including yellow (YFP), blue and cerulean variants.
Where it matters for labs
These numbers translate into three checks before an experiment:
- Excitation source. EGFP’s excitation maximum is 488 nm, while wild-type GFP absorbs most strongly at 395 nm. A light source or filter set matched to one variant is not automatically matched to the other.
- Emission window. Both proteins emit near 507 nm, so the emission band collected must cover that region.
- Tag mass and oligomerisation. A fusion adds about 27 kDa, and EGFP’s weak-dimer behaviour on FPbase is a reason to read how a given construct was validated when the target protein itself forms complexes.
GFP-family proteins also sit behind several methods covered elsewhere on CASRAI: the fluorophores guide covers brightness and photobleaching, FRET microscopy uses fluorescent-protein pairs, FRAP measures mobility of fluorescent fusions, calcium imaging relies on GFP-derived GCaMP indicators. For antibody-based detection of fluorescence signals see immunofluorescence controls.
Common misunderstandings
GFP needs a cofactor or substrate
The UniProt annotation says chromophore maturation requires nothing other than molecular oxygen. Oxygen is needed, so a fully anoxic sample will not mature the chromophore.
GFP and EGFP are interchangeable
Their excitation maxima differ (395 nm versus 488 nm in the sources above), so an optical setup must be chosen for the variant actually used.
GFP is a small tag
At about 27 kDa it is larger than many proteins people tag with it.
GFP is one fixed protein
The name covers the original jellyfish protein and a large engineered family, including yellow and blue variants.
FAQ
What is GFP protein?
GFP is green fluorescent protein, a 238-amino-acid protein from Aequorea victoria that forms its own fluorescent chromophore and emits green light near 507-510 nm.
What is the size of GFP protein?
Wild-type GFP is 238 amino acids and 26,886 Da (about 27 kDa) according to UniProt P42212.
What wavelengths does GFP excite and emit at?
Wild-type GFP has a major absorbance peak at 395 nm and a smaller one at 470 nm, with emission peaking at 507-510 nm (UniProt P42212). EGFP has an excitation maximum of 488 nm and an emission maximum of 507 nm (FPbase).
Is GFP the same as EGFP?
No. EGFP is an engineered variant derived from the Aequorea victoria protein, with a different excitation maximum (488 nm versus 395 nm) and a reported weak-dimer state, whereas UniProt annotates wild-type GFP as a monomer.
Does GFP need a cofactor to fluoresce?
UniProt states that chromophore maturation requires nothing other than molecular oxygen.
Sources
- UniProt Swiss-Prot P42212, Green fluorescent protein, Aequorea victoria, uniprot.org/uniprotkb/P42212 (checked 2026-10-29).
- RCSB Protein Data Bank entry 1EMA, rcsb.org/structure/1EMA (checked 2026-10-29).
- FPbase, EGFP, fpbase.org/protein/egfp (checked 2026-10-29).
