13xlexAop regulatory sequences drive expression of an 'EMcapsulin' that produces robust electron microscopy (EM) contrast with nanoscale precision and using standard EM fixation and staining protocols. The encoded protein consists of three copies of Metallothionein (encoded by the mouse Mt3 gene, the SmtA gene of Synechococcus sp. PCC7942 and the EC1 gene of Triticum aestivum respectively) fused to the N-terminal end of the Bthe\enc coding sequence, with a Tag:FLAG tag and a nuclear localization signal at the C-terminal end of the Bthe\enc sequence. The Bthe\enc sequence self-assembles into a hollow protein nanosphere with the triple Metallothionein peptide facing the lumen of the porous protein shell. The triple Metallothionein peptide can bind heavy metals, resulting in an annular ring of robust EM contrast.