20xUAS 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 a single copy of Metallothionein (encoded by the mouse Mt3 gene) fused to the N-terminal end of the Mxan\encA coding sequence, with a Tag:FLAG tag and a nuclear localization signal at the C-terminal end of the Mxan\encA sequence. The Mxan\encA sequence self-assembles into a hollow protein nanosphere with the Metallothionein peptide sequence facing the lumen of the porous protein shell. The Metallothionein peptide can bind heavy metals, resulting in an annular ring of robust EM contrast.