Coronavirus IBV: Structural Characterization of the Spike Protein

The spike protein (S; surface projection) of avian infectious bronchitis virus (IBV) strain M41 comprises two glycopolypeptides, S1 (mol. wt. 90 X 10(3] and S2 (mol. wt. 84 X 10(3], in equimolar proportions. The apparent mol. wt. of S was calculated as 354 (+/- 17) X 10(3) following co-sedimentation with catalase in sucrose gradients. Incubation of radiolabelled IBV with urea resulted in the removal of most S1, but none of S2, from the virus particle. A similar result was obtained using low concentrations of SDS, although some nucleocapsid, but not matrix, protein was also released. 2% SDS alone was as effective as 2% SDS plus 2% 2-mercaptoethanol for the separation of S1 and S2 prior to SDS-polyacrylamide gel electrophoresis. Dithiothreitol did not remove S from virions but did decrease the buoyant density of the virus from 1.18 g/ml to 1.16 g/ml, and changed the configuration of S. It is concluded that IBV S protein is an oligomer comprising two copies of each of S1 and S2, although the possibility that there are three copies of each glycopolypeptide cannot be discounted. S is attached to the membrane by S2, while S1 has little or no contact with the membrane and may form the major part of the bulbous end of S. Interpeptide disulphide bonds do not occur in S, and the association of S1 and S2 is weak.