Biochemical and Immunological Analysis of Discontinuous Epitopes in the Family of Human Cytomegalovirus Glycoprotein Complexes Designated gC-I

Abstract

The envelope of human cytomegalovirus contains a family of disulphide-linked glycoprotein complexes designated gC-I which contain two glycoproteins of 52,000 Mr (gp52) and 93,000 to 130,000 Mr (gp93-130). Epitopes recognized by several of our gC-I gp52-specific monoclonal antibodies (MAbs) were previously assigned to three domains based on reactivity with gC-I in a competitive binding assay. In this report, we have used additional gC-I MAbs to characterize three distinct discontinuous epitopes in the gC-I complexes. Two of these epitopes were in Domain I and one in Domain III. These epitopes were resistant to proteolysis, heat denaturation and SDS treatment. However, the discontinuous epitopes were lost after reduction of disulphide bonds. After digestion of gC-I complexes with chymotrypsin, two fragments of 43,000 (43K) Mr and 34,000 (34K) Mr were obtained which contained all discontinuous and continuous epitopes recognized by our gp52 MAbs. The Mr of these fragments could not be reduced further by longer digestion or by use of other proteases such as trypsin or pronase. The 43K fragment contained N-linked oligosaccharides not detected in the 34K fragment. These oligosaccharides may have prevented a complete proteolytic digestion so that the 34K fragment was not always obtained. It was established that 80 to 90% of the mass of these fragments was contributed by gp52. Thus the discontinuous epitopes were composed primarily of gp52 and not gp93-130.