  We propose a mesoscale model structure for the coronavirus nucleocapsid,
assembled from the high resolution structures of the basic building blocks of
the N-protein, CryoEM imaging and mathematical constraints for an overall
quasi-spherical particle. The structure is a truncated octahedron that
accommodates two layers: an outer shell composed of triangular and quadrangular
lattices of the N-terminal domain and an inner shell of equivalent lattices of
coiled parallel helices of the C-terminal domain. The model is consistent with
the dimensions expected for packaging large viral genomes and provides a
rationale to interpret the apparent pleomorphic nature of coronaviruses.
