Structural proteins of bovine coronavirus strain L9: effects of the host cell and trypsin treatment

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Abstract
The polypeptide profile of the cell-adapted strain of bovine coronavirus (Mebus BCV-L9) is remarkably affected by the host cell and trypsin. We compared the structural proteins of virus purified from different cell lines and found cell-dependent differences in the virus structure. BCV was purified from four clones of human rectal tumour cells (HRT-18): 3 F3, D 2, 3 E 3, and 4 B 3. The structural profiles of BCV propagated in clones 3 E 3 and 3 F 3 were identical, consisting of proteins with molecular weights of 185, 160, 140, 125, 110, 100, 52, 46, 37, 31–34, and 26–28 kilodaltons (kd). BCV purified from clone D2 lacked the 100 kd species, and clone 4 B 3 yielded virus lacking the 46 kd protein. We compared the structures of BCV propagated in HRT-18 cells [BCV(HRT-18)] and virus raised in bovine fetal spleen cells [BCV(D 2 BFS)]. The concentration of the 185 kd protein was higher in BCV (D 2BFS), and it also contained a 200 kd species. Protein profiles of in vitro trypsin treated and untreated BCV(HRT-18) differed only under reducing conditions, suggesting that trypsin cleavage sites are located within disulfide-linked regions of affected proteins. Propagation of BCV in D 2 BFS cells in the presence of trypsin resulted in cleavage of the 185 kd protein and a concommitant increase of the 100 kd protein. Activation of the fusion function probably depends on this cleavage process because fusion of BCV-infected D 2 BFS cells is trypsin dependent.