Encapsulins are a novel class of protein nanocages isolated from bacteria. They have emerging as promising nanoplatforms since they specifically confine functional and fully folded protein-based cargo, such as enzyme and a ferritin-like protein, in that way being recognized as a bacterial semiorganelle. Encapsulin purified from the metal-reducing alkaliphilic bacterium (Alkaliphilus Metalliredigens) is a new protein-based vessel for ferritin-like protein in its cavity. Here, we characterize their structure of both native and protein loaded encapsulins using Cryo-electron microscopy. We reveal the assembly principles of a T=1 shell topology and its catalytic ferroxidase cargo.