Intra- and Intermolecular Disulfide Bonds of theGP 2b Glycoprotein of Equine Arteritis Virus: Relevance forVirus Assembly andInfectivity

Abstract

Equine arteritis virus (EAV) is an enveloped, positive-strand RNA virus belonging to the family Arteriviridae of the order Nidovirales . EAV virions contain six different envelope proteins. The glycoprotein GP 5 (previously named G L ) and the unglycosylated membrane protein M are the major envelope proteins, while the glycoproteins GP 2b (previously named G S ), GP 3 , and GP 4 are minor structural proteins. The unglycosylated small hydrophobic envelope protein E is present in virus particles in intermediate molar amounts compared to the other transmembrane proteins. The GP 5 and M proteins are both essential for particle assembly. They occur as covalently linked heterodimers that constitute the basic protein matrix of the envelope. The GP 2b , GP 3 , and GP 4 proteins occur as a heterotrimeric complex in which disulfide bonds play an important role. The function of this complex has not been established yet, but the available data suggest it to be involved in the viral entry process. Here we investigated the role of the four cysteine residues of the mature GP 2b protein in the assembly of the GP 2b /GP 3 /GP 4 complex. Open reading frames encoding cysteine-to-serine mutants of the GP 2b protein were expressed independently or from a full-length infectious EAV cDNA clone. The results of these experiments support a model in which the cysteine residue at position 102 of GP 2b forms an intermolecular cystine bridge with one of the cysteines of the GP 4 protein, while the cysteine residues at positions 48 and 137 of GP 2b are linked by an intrachain disulfide bond. In this model, another cysteine residue in the GP 4 protein is responsible for the covalent association of GP 3 with the disulfide-linked GP 2b /GP 4 heterodimer. In addition, our data highlight the importance of the correct association of the minor EAV envelope glycoproteins for their efficient incorporation into viral particles and for virus infectivity.