This is the structure of the 20S core particle of the human 26S proteasome, based on the structure published by Huang et al. (Nature Structural & Molecular Biology 23, 778–785 (2016) doi:10.1038/nsmb.3273).

The PDB file representing the structure determined by the researchers was edited to remove the 19S regulatory subunits and the surface rendered using Chimera (http://www.cgl.ucsf.edu/chimera/), and exported as an .STL file.