Crystallization and preliminary X-ray crystallographic analysis of a nonstructural protein 15 mutant from Human coronavirus 229E.

Nonstructural protein 15 (nsp15), also called endoribonuclease, is a gene product of open reading frame 1b (ORF 1b) in coronaviruses. It is an important enzyme in the transcription/replication process involved in discontinuous negative-strand RNA synthesis. In this work, mutants of nsp15 from Human coronavirus 229E (HCoV-229E) were made based on structural analysis of the homologous nsp15s in Severe acute respiratory syndrome coronavirus (SARS-CoV) and Mouse hepatitis virus (MHV). The I26A/N52A mutant of nsp15 was overexpressed, purified and crystallized, and this mutant led to a trimeric form rather than hexamers or monomers. Crystals of trimeric nsp15 were obtained by the hanging-drop vapour-diffusion method using polyethylene glycol as a precipitant and diffracted to 2.5 Å resolution. The crystals belonged to space group C2221, with unit-cell parameters a = 85.9, b = 137.5, c = 423.1 Å, α = β = γ = 90°.