| Clonable contrasting agents for light microscopy, such as green fluorescent protein have revolutionized biology, but no such agents have been developed for transmission electron microscopy (TEM). Instead, TEM labels for biological studies often rely on the chemical attachment of man-made, electron dense, metal nanoclusters to proteins of interest in order to distinguish these proteins from others within cells or macromolecular complexes. As an attempt to develop a novel clonable contrasting agent for TEM, metallothionein, a small metal binding protein, reacted with aurothiomalate, an anti-arthritic gold compound, was evaluated. An examination of this reaction with an excess amount of aurothiomalate suggests metallothionein accumulates single gold atoms in a manner more akin to the formation of man-made gold clusters rather those normally witnessed with other proteins. Furthermore, TEM visualization of concatenated metallothionein fusion proteins incubated with gold show rather uniform electron-dense particles of almost equivalent size and density to man-made TEM labels. The results presented suggest gold bound metallothionein may act as the highly sought, clonable TEM label. In addition, secondary work demonstrated that metallothionein, when fused to another protein, can be used to specifically isolate these fusion proteins via affinity chromatography most likely due to metal lothionein's unique metal binding ability. Hence, not only may metallothionein act as the desired TEM label but also as a protein purification tag. |
