(A) Schematic of the APEX-Gold method. Cells were transfected with Cavin4-APEX2 (B,D,E) (control light microscopy experiments are described in Figure 1—figure supplement 5) or LifeAct-APEX2 (C,F), fixed, treated with diaminobenzidine (DAB), and then incubated with Ag/Au reagents in the presence of gum arabic. (B,D,E) Low (B) and higher (D,E, inset in B) magnification views of caveolae labelling. (C) Labelled actin filaments. (F) Optical slice projection through tomogram of LifeAct-APEX2 expressing cells. APEX-Gold particulate reaction product can be observed tightly associated with and throughout the actin bundles in three dimensions. Note the uniform gold label, the lack of background, and high signal to noise. PM, plasma membrane; CCP, clathrin-coated pit. Bars, B, 2 µm (inset 500 nm); C, 1 µm; D, 500 nm; E, 1 µm; F, 500 nm.