1.A.120. The Coronaviral Double Membrane Pore Complex (CoV-DMPC) Family
Coronaviral RNA is replicated in double membrane vesicles (DMVs) by the replicatioin organelle (RO), but the RNA must exit the DMV and enter the cytoplasm in order to be translated into protein. Only the +RNA strand and the mRNAs need to be exported. There appears to be a molecular pore complex that spans both membranes of the DMV (Wolff et al. 2020). The pore has an internal diameter of about 260 nm (based on cryo EM) and therefore can probably accomodate both ssRNA and dsRNA. It is crown-shaped with the crown projecting into the cytoplasm where it probably interacts with nsp4 and the viral nucleocapsid (N) protein. The virus encodes three integral membrane proteins, nsp3, nsp4 and nsp6. nsp3 is large with 2309 aas and a size of about 222 kDas. Although it has several (up to 6) regions that are hydrophobic enough to span the membrane, Wolff et al. 2020 claim that it has 2 TMSs; still to span two membranes, it may need 4 TMSs. nsp3 is known to interact with nsp4, and this interaction drives membrane pairing and determines DNV biogenesis and morphology. nsp4 is 56 kDa in size and has 4 putative TMSs; nsp6 is 33 kDa in size with 6 putative TMSs (Wolff et al. 2020). nsp3 has a ubiquitin (Ubl1) domain that is believed to modulate RNA exit to the cytoplasm and may interact with the nucleocapsid (N) protein to stabilze the RNA in the cytoplasm, both for translation and for packaging into the viral particles (the virions). There are likely to be additional proteins, possible both viral and host proteins, that are associated with the pore complex, but only these two proteins will be included in TC entry, TC# 1.A.20.1.1. Coronavirus non-structural proteins, Nso3/6, derive from the polyproteins of various coronaviruses, and usually have 200 - 300 aas, often with about 8 TMSs. They have been proteolytically processed from polyproteins of 4000 to 8000 aas. It is not always clear whether coronaviral proteins referred to in the protein databases are the processed or unprocessed products of the polyproteins.
The reaction believed to be catalyzed by the double membrane pore is:
RNA (lumen of the DMV) → RNA (cytoplasm)