Partial deletion in the spike endodomain of mouse hepatitis virus decreases the cytopathic effect but maintains foreign protein expression in infected cells

Mouse hepatitis virus (MHV) produces a series of subgenomic RNAs for viral protein expression. As a prototype coronavirus, MHV has been explored extensively and is often used to express foreign proteins. Previously, a 13-residue deletion in the MHV spike (S) protein endodomain was found to reduce syncytium formation dramatically while inhibiting virus replication slightly. In this study, the effects of the S mutation on MHV infectivity and foreign protein expression were further examined in rat or mouse L2, NIH/3T3 and Neuro-2a cells. The replacement of the MHV 2a/haemagglutinin-esterase gene with a membrane-anchored protein hook (HK) and replacement of gene 4 with EGFP did not change the adaptability and cytopathology of recombinant viruses in these cells. However, the cytopathic effect of the recombinants with the partial S deletion was reduced significantly in these cells. The replication and foreign protein expression of the S-mutated recombinants were found to be more efficient in L2 cells than in Neuro-2a and NIH/3T3 cells. Meanwhile, the distribution patterns of HK and EGFP expressed by the recombinant viruses were similar to those in cells transfected with a eukaryotic expression vector. These results suggest that the partial deletion in the S endodomain may increase the usefulness of MHV as a viral vector by attenuation and maintaining foreign protein expression.