Green Fluorescent Protein (GFP) is a naturally occurring biomolecule known for its ability to emit a vibrant green light when exposed to certain wavelengths of light. Its inherent luminescence makes it a valuable tool across various scientific disciplines. GFP functions as a self-contained light source within biological systems, requiring only light in the blue to ultraviolet range to become fluorescent.
The Source of Green Light
The origins of GFP trace back to the bioluminescent jellyfish Aequorea victoria, found in the cold waters of the Pacific Ocean. In the early 1960s, Osamu Shimomura, a Japanese marine biologist, first isolated and characterized GFP while studying the jellyfish’s light-emitting properties. He initially sought to understand the blue light produced by another protein, aequorin, and unexpectedly discovered GFP as a byproduct due to its bright fluorescence. The jellyfish produces both aequorin and GFP; GFP absorbs the blue light from aequorin and re-emits it as green light, contributing to the jellyfish’s natural green glow when stimulated.
Unveiling the Glow Mechanism
The ability of GFP to glow green stems from a unique internal structure called a chromophore. This chromophore is formed through a spontaneous internal reaction involving three specific amino acids: serine at position 65, tyrosine at position 66, and glycine at position 67, located within the protein’s beta-barrel structure. When GFP is exposed to blue or ultraviolet light, the chromophore absorbs this energy, exciting its electrons to a higher energy state.
Following excitation, the chromophore quickly releases this absorbed energy by emitting light at a longer wavelength, specifically around 509 nanometers, which falls within the green part of the visible spectrum. This process, known as fluorescence, occurs without the need for external cofactors or complex enzymatic reactions, making GFP self-sufficient. The stable cylindrical beta-barrel structure of the protein encapsulates and protects the chromophore, ensuring consistent fluorescence under varying cellular conditions.
Illuminating Biological Processes
GFP has transformed biological research by allowing scientists to visualize dynamic cellular and molecular processes in living systems. Researchers can genetically engineer organisms, ranging from bacteria and yeast to plants and mammals, to produce GFP. This is typically achieved by fusing the gene for GFP with the gene for a protein of interest, creating a “fusion protein” that glows green wherever the target protein is located or expressed.
This genetic engineering enables real-time observation of biological phenomena that were previously difficult to study. Scientists use GFP for various applications, including:
Tracking gene expression, revealing when and where specific genes are active
Observing protein localization, showing where proteins reside and move within cells
Monitoring cellular movement during processes like development or immune responses
Identifying specific cell types
Tracing cell lineages during development
Monitoring the progression of diseases
GFP’s introduction has allowed for non-invasive imaging, making it a valuable tool that continues to drive discoveries in cell biology, neuroscience, and medicine.