ANTIGENIC STRUCTURE OF ENVELOPE GLYCOPROTEIN E1 OF HOG-CHOLERA VIRUS

Envelope glycoprotein E1 (gp51 to gp54) is the most antigenic protein of hog cholera virus or classical swine fever virus (CSFV). Four antig enic domains, A to D, have been mapped on E1 with a panel of monoclona l antibodies (MAbs) raised against CSFV strain Brescia. The boundaries of these domains hare been established by extensive studies on bindin g of MAbs to transiently expressed deletion mutants of E1 (P.A. van Ri jn, E.J. J. de Meijer, H.G.P. van Gennip, and R.J.M. Moormann, J. Gen. Virol. 74:2053-2060, 1993). In this study, we used neutralizing MAbs of domains A, B, and C to isolate MAR-resistant mutants (MAR mutants) of CSFV strain Brescia and Chinese vaccine strain (''C''). The El gene s of MAR mutants were cloned in a eukaryotic expression vector, and th e effects of MAR mutations on epitopes were studied, with a panel of 1 9 MAbs by immunostaining of COS1 cells transiently expressing these mu tant E1s. Except for the MAR mutation Cys-->Arg at position 792, which abolished binding of all MAbs of domains A and D, amino acid substitu tions affected only MAbs belonging to the same domain as the MAbs used to select the MAR mutant. However, a MAR mutation in a particular dom ain did not per se abolish binding of all MAbs recognizing that domain . Furthermore, MAR mutants possessed conservative as well as nonconser vative amino acid substitutions. To investigate the significance of a secondary. structure for the binding of MAbs, all cysteine residues in the N-terminal antigenic part of E1 were mutated to serine. We found that the cysteines at positions 693 and 737 were essential for binding by MAbs of domains B and C, whereas those at positions 792, 818, 828, and 856 appeared to be essential for the binding of most MAbs of doma ins A and D. These results fully comply with the previously proposed t wo-unit structure of the N-terminal half of E1. One unit consists of a ntigenic domains Il and C, whereas the other unit consists of the high ly conserved domain A and domain D. We conclude that the first six cys teines are critical for the correct folding of E1. A model of the anti genic structure of E1 is presented and discussed.