Competitive Fitness in Coronaviruses Is Not Correlated with Size or Number of Double-Membrane Vesicles under Reduced-Temperature Growth Conditions

Al-Mulla, Hawaa M. N.; Turrell, Lauren; Smith, Nicola M.; Payne, Luke; Baliji, Surendranath; Züst, Roland; Thiel, Volker; Baker, Susan C.; Siddell, Stuart G.; Neuman, Benjamin W.

The effect of the Brts31 mutation in reducing DMV formation may be explained by the essential role of nsp3 in organelle formation (17) . However, we could not find a simple explanation for finding that the Brts105 mutation in nsp14, located over 15,000 nucleotides (nt) downstream of the Brts31 mutation, in a domain which had not previously been shown to be essential for DMV synthesis, also reduced organelle formation. While it is possible that the N7 methyltransferase activity of nsp14 is involved in an early step of DMV formation, a more likely explanation is that the mutation alters long-range protein folding or protein processing that occurs upstream of DMV formation, as observed previously for mutations in nsp10 and nsp3 (14, 27) . However, further work will be needed to study the effects of the Brts105 mutation on the function of other essential replicase proteins such as nsp5 (14, 27) or protein complexes such as the nsp10-nsp14-nsp16 RNA cap methylation complex (43) . Notwithstanding, the results presented here constitute the first demonstration of a functional linkage between nsp14 and the DMV scaffold proteins.