N-TERMINAL SEQUENCE-ANALYSIS OF EQUINE HERPESVIRUS-1 GLYCOPROTEIN-D AND GLYCOPROTEIN-B AND EVIDENCE FOR INTERNAL CLEAVAGE OF THE GENE-71 PRODUCT

Signal cleavage sites of equine herpesvirus 1 (EHV-1) glycoproteins D and B (gD and gB) and an endoproteolytic cleavage site of EHV-1 gB wer e determined by N-terminal amino acid sequencing and compared with kno wn cleavage sites of homologues in other herpesviruses. Signal cleavag e of EHV-1 go occurred between Arg(35) and Ala(36) in a region of basi c amino acids resembling the endoproteolytic cleavage sites of viral g lycoproteins, nine amino acids downstream of the predicted site, while EHV-1 gB was cleaved as predicted between Ala(85) and Val(86). Endopr oteolytic cleavage of EHV-1 gB occurred between Arg(548) and Ala(549), 28 amino acids downstream of the cleavage site predicted from conserv ed sequences of other herpesvirus gB homologues. One interpretation of these data is that EHV-1 gB is cleaved internally at both sites, a po ssibility which was supported by the apparent molecular masses of the unglycosylated gB subunits produced in the presence of tunicamycin. Th is double cleavage would release a stretch of amino acids which is not present in sequenced gB molecules of other herpesviruses. Experiments with glycosylation inhibitors indicated that cleavage of EHV-1 gB can occur in the absence of glycosylation. N-terminal sequencing also det ermined that a 42 kDa EHV-1 glycoprotein was a product of internal cle avage of the protein encoded by gene 71. Staggered endoproteolytic cle avage after adjacent arginine residues 506 and 507 separates the 42 kD a C-terminal subunit containing all the cysteine residues from the ser ine and threonine rich N-terminal region.