Electron microscopy enables high-resolution visualisation of molecules inside cells

The glowing green molecule known as green fluorescent protein (GFP) has revolutionised molecular biology. When GFP is attached to a particular protein inside a cell, scientists can easily identify and locate it using fluorescence microscopy. However, GFP can't be used with electron microscopy, which offers much higher resolution than fluorescence microscopy.

Chemists from MIT have now designed a GFP equivalent for electron microscopy - a tag that allows scientists to label and visualise proteins with unprecedented clarity.

''With things that may appear only a few pixels across by fluorescence microscopy - for example, a mitochondrion - you can't make out any of the internal features. But with electron microscopy it's very easy to discern the intricate internal structures,'' says Jeff Martell, a graduate student in chemistry at MIT and lead author of a paper describing the new tag in the 21 October online edition of Nature Biotechnology.

The new tag could help scientists pinpoint the locations of many cell proteins, providing new insight into those proteins' functions, according to the researchers.

Improving on nature
Dubbed APEX, the new tag is similar to naturally occurring proteins that have been tried as imaging labels for electron microscopy. Horseradish peroxidase (HRP) is one commonly used tag, but it works only in a few compartments of a cell. Other recently developed tags work throughout a cell but are technically challenging to use because they require light to be shined on the sample and oxygen to be bubbled through it.

To improve on these methods, the researchers started with a protein similar to HRP, called ascorbate peroxidase (APX). APX is more versatile than HRP because it can function within a cell's cytosol, in the main cavity of a cell.