sars5

The most abundant structural protein in the virion is sars5, the matrix (M) protein. This has a similar three membrane-helix topology to sars3a. It is conceivable that M and sars3a are structurally analogous proteins, and that a small population of M proteins could be replaced by sars3a within the virion. The M protein interacts with the N protein, via a 12-residue segment (residues 194-205) and probably links spike with the ribonucleoprotein core. The M protein is thought to bind N, S and E and escorts them to the ER-Golgi intermediate compartment – the site of budding for the SARS-CoV. The details of this interaction between M and E remain unclear. The structure of M is also not known, particularly what fold the soluble domain of the protein adopts. M has also been implicated in discriminating between genomic and sub-genomic RNAs during virion packaging, although details of this process are not known.

 

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