Artificial protein cages - inspiration, construction, and observation.

Protein cages are hollow, often spherical, protein structures. They are scientifically interesting for reasons including their capability to serve as protective containers for delivering medically useful cargoes to cells. Design and construction of artificial protein cages is a powerful strategy enabling them to be endowed with bespoke properties not seen in natural forms. To this end, structural studies are a vital tool: Structural analyses of naturally existing protein cages can provide an inspiration for artificial designs while determining structures of artificial proteins can confirm that they match expected designs and cryo-EM is now the tool of choice to achieve this. In this review we describe how natural protein cage structures can inform the design of artificial versions and how, in turn, these can exceed the limitations of their natural counterparts.