RNA replication and transcription machinery is an important drug target for fighting against

coronavirus. Non-structure protein nsp8 was proposed harboring primase activity. However,

the RNA primer synthesis mechanism of nsp8 is still largely unknown. Here, we purified

dimer and tetramer forms of SARS-CoV-2 nsp8. Combined with dynamic light scattering,

small-angle neutron scattering and thermo-stability analysis, we found that both dimer and

tetramer become loosened and destabilized with decreasing salt concentration, and the dimer

form is more stable than the tetramer form. Further investigation showed that nsp8 dimer

and tetramer can undergo phase separation but exhibit different phase separation behaviors.

Nsp8 dimer can form liquid-like droplets in the buffer with a low concentration of NaCl; phase

separation of nsp8 tetramer depends on the assistance of RNA. Our findings on different

phase separation behaviors of nsp8 dimer and tetramer may provide insight into the functional

studies of nsp8 in coronavirus.