A Systemic and Molecular Study of Subcellular Localization of SARS-CoV-2 Proteins

Coronavirus possesses the largest RNA genome among all the RNA viruses. Its genome encodes about 29 proteins. Most of the viral proteins are non-structural proteins (NSP) except envelop (E), membrane (M), nucleocapsid (N) and Spike (S) proteins that constitute the viral nucleocapsid, envelop and surface. We have recently cloned all the 29 SARS-CoV-2 genes into vectors for their expressions in mammalian cells except NSP11 that has only 14 amino acids (aa). We are able to express all the 28 cloned SARS-CoV-2 genes in human cells to characterize their subcellular distributions. The proteins of SARS-CoV-2 are mostly cytoplasmic but some are both cytoplasmic and nuclear. Those punctate staining proteins were further investigated by immunofluorescent assay (IFA) using specific antibodies or by co-transfection with an organelle marker-expressing plasmid. As a result, we found that NSP15, ORF6, M and ORF7a are related to Golgi apparatus, and that ORF7b, ORF8 and ORF10 colocalize with endoplasmic reticulum (ER). Interestingly, ORF3a distributes in cell membrane, early endosome, endosome, late endosome and lysosome, which suggests that ORF3a might help the infected virus to usurp endosome and lysosome for viral use. Furthermore, we revealed that NSP13 colocalized with SC35, a protein standing for splicing compartments in the nucleus. Our studies for the first time visualized the subcellular locations of SARS-CoV-2 proteins and might provide novel insights into the viral proteins biological functions.