EPITOPE MAPPING IDENTIFIES AN EXPOSED LOOP BETWEEN THE UNIQUE AMINO-DOMAINS AND CONSERVED CARBOXY-DOMAINS OF THE LARGE SUBUNIT OF HERPES-SIMPLEX VIRUS TYPE-1 RIBONUCLEOTIDE REDUCTASE

The large subunits of herpes simplex virus types 1 and 2 ribonucleotid e reductases contain unique amino-terminal regions comprising 311 and 318 residues respectively, which are not found in ribonucleotide reduc tases from other sources. We report the mapping of the epitope recogni zed by monoclonal antibody 1026, which is specific for the large subun it (R1) of HSV-1, and then deduce the structural relationship of the a mino-terminal region of R1 with the rest of the protein. A panel of 10 fusion proteins containing sequences spanning the entire R1 subunit w ere constructed. They were used together with proteolytic fragments of R1 and several synthetic peptides to show that the epitope is discont inuous and appears to be a loop structure centred on a previously loca ted trypsin-sensitive site at residue 305. The existence of the loop w as suggested by the observation that reactivity of the antibody with R 1 could be blocked by peptides corresponding to residues 289 to 303 an d 308 to 313 which flank the trypsin-sensitive site. Our results sugge st that the unique amino-terminal region of R1 consists of a structura lly distinct domain which is linked to the conserved carboxy region by an exposed loop.