Coronavirus nucleocapsid proteins assemble constitutively in high molecular oligomers

Cong, Yingying; Kriegenburg, Franziska; de Haan, Cornelis A. M.; Reggiori, Fulvio

One important finding of our study is that cytoplasmic MHV N protein forms high molecular weight oligomers in infected cells. Similarly to the recombinant protein, we could not detect monomers, dimers or small multimers on glycerol gradients (Fig. 2b) suggesting that after synthesis, MHV N protein rapidly assembles into oligomers. It is easy to imagine that, based on our in vitro data, this is very likely also the case for the SARS-CoV N protein. Since all CoV N proteins have an identical modular organization 21 and have the N1b, N2a and N2b domains, oligomerization could be a characteristic that all of them possess. It has been suggested that gRNA promotes MHV N protein self-interaction because the association process was partially or largely susceptible to RNase A treatment 11, 35 . Another study, however, reached the opposite conclusion 19 , which is also supported by structural biology studies where N protein multimers have been detected and analyzed in preparations that do not contain RNA 12, 22-24, 27, 29, 32, 36, 49 . Our data showing that both in vivo and in vitro N protein oligomerization does not depend on its binding to gRNA are in agreement with this latter conclusion. It cannot be excluded, however, that association to gRNA could promote further N protein oligomerization. This could explain the partial discrepancy with the study showing that RNase A treatment interferes with the binding between the N protein and full-length N protein or N1b/NTD domain, but not with the N2b/CTD region 11 .