4.4 Expression of fluorescent proteins

Some dyes can be used in living cells, but one of the most powerful and widely used labelling techniques in live cultured cells, and even whole organisms, is to genetically modify them to express a fluorescent protein. Interestingly, certain animals naturally produce fluorescent proteins as part of their defence against predators. One well-known example is the jellyfish Aequorea victoria, which produces GFP, a molecule that you have encountered earlier in the course (Figure 13a).

Scientists can take the gene that encodes GFP and fuse it to a gene of interest and then introduce this modified gene into cells (Figure 13b) or even whole animals (Figure 13c and d). The resulting GFP-fusion protein can be visualised in real time under a fluorescence microscope, allowing researchers to track where the protein is located inside the living cell. In most cases, the fusion protein behaves like the original protein, maintaining its normal function and proper localisation within the cell.

Figure 13 GFP expression. (a) The jellyfish Aequorea victoria naturally expresses GFP in the light-emitting organs at the bottom of its bell. (b) HeLa cells expressing GFP on their ER. (c) GFP expressed in neurons of the worm Caenorhabditis elegans. (d) A mouse expressing GFP.

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The figure consists of four parts, Figures 13a to d, that illustrate fluorescence micrographs of GFP expression in different organisms.

Figure 13a: a micrograph shows the bell-shaped jellyfish Aequorea victoria expressing GFP. The light-emitting organs of the jellyfish express GFP and emit fluorescence at the bottom of its bell.

Figure 13b: a micrograph shows spindle-shaped HeLa cells with the nucleus at the centre surrounded by a network of endoplasmic reticulum. The endoplasmic reticulum of the HeLa cells expresses GFP and emits green fluorescence. The scale bar reads 20 micrometres.

Figure 13c: a micrograph shows three spindle-shaped worms Caenorhabditis elegans expressing GFP in neuron, which emit green fluorescence.

Figure 13d: a micrograph shows a mouse expressing GFP. The ears, eyes, and front part of the mouth express GFP and emit green fluorescence.

Figure 13 GFP expression. (a) The jellyfish Aequorea victoria naturally expresses GFP in the light-emitting organs at the bottom of its ...

You have now learned about the different types of microscopy, the principles of fluorescence and how cellular structures using fluorescence. In the next section you will briefly explores why fluorescence microscopy is considered such a versatile technique in cell biology.