The C-terminal Domain of SARS-CoV-2 nsp8 is a Molten Globule in the Absence of Binding Partners

The coronavirus genome is transcribed by a replication-transcription complex (RTC) containing the RNA polymerase plus additional cofactors. The cofactor nsp8 is an important component of the RTC in both alpha and betacoronaviruses required for nsp12 polymerase activity, complex stability, and recruitment of other RTC cofactors. Here we use NMR and other biophysical methods to characterize the structural features and oligomeric state of full-length nsp8 in solution. We show that the C-terminal domain of nsp8 has molten-globule like intrinsic disorder, while the N-terminal domain retains its folded structure in the absence of binding partners. Our data also shows a concentration-dependent association of nsp8 into dimers and possibly tetramers, but not larger molecular weight species. Upon binding nsp7, the C-terminal domain of nsp8 folds into a well-defined conformation consistent with available structures of the complex, while the linker region connecting the N- and C-terminal domains remains disordered.