Two glycoproteins are produced from the rotavirus neutralization gene

The major neutralization antigen of rotaviruses is an outer capsid glycoprotein, VP7, with an apparent molecular weight of 38,000 (38K). The simian rotavirus SA11 genome segment 9, which codes for VP7, contains two in-phase initiation codons, each of which is followed by a sequence that codes for a region of hydrophobic amino acids. We have determined that this gene is functionally bicistronic by analyzing the synthesis of VP7 in SA11-infected cells and in cell-free translation systems programmed with hybrid-selected, segment 9 specific mRNA and dog pancreatic microsomes. The translation of hybrid-selected gene 9 mRNA in wheat germ extracts yielded two distinct polypeptides of molecular weights 37K and 35.3K. In vitro translation in the presence of microsomes yielded one diffuse band of 38K that was converted into the 37K and 35.3K precursor bands by digestion with endoglycosidase H. Studies with a variant of SA11 that lacks the glycosylation site in VP7 confirmed these precursor-product relationships and extended them by indicating that the glycoprotein produced by translation from the first AUG contained a cleaved signal sequence whereas the glycoprotein produced by translation from the second AUG contained an uncleaved signal sequence. Immunoprecipitation with monospecific anti-VP7 serum and improved gel electrophoresis conditions allowed us to show that both VP7s were expressed at similar times in infected cells and both were found in purified virus particles of several different rotavirus strains. Whether these two VP7 glycoproteins are functionally distinct remains to be determined.