Molecular mapping of virus-infected cells with immunogold and metal-tagging transmission electron microscopy

Transmission electron microscopy (TEM) has been essential to study virus-cell interactions. The architecture of viral replication factories, the principles of virus assembly and the components of virus egress pathways are known thanks to the contribution of TEM methods. Specially, when studying viruses in cells, methodologies for labeling proteins and other macromolecules are important tools to correlate morphology with function. In this review, we present the most widely used labeling method for TEM, immunogold, together with a lesser known technique, metal-tagging transmission electron microscopy (METTEM) and how they can contribute to study viral infections. Immunogold uses the power of antibodies and electron dense, colloidal gold particles while METTEM uses metallothionein (MT), a metal-binding protein as a clonable tag. MT molecules build gold nano-clusters inside cells when these are incubated with gold salts. We describe the necessary controls to confirm that signals are specific, the advantages and limitations of both methods, and show some examples of immunogold and METTEM of cells infected with viruses.