Mutational analysis of the envelope protein of spleen necrosis virus

Spleen necrosis virus (SNV) is an amphotropic type C retrovirus originally isolated from a duck. The envelope protein is related to that of type D retroviruses, and SNV appears to use the same receptor as do simian retroviruses. However, little is known about envelope-receptor interactions of SNV. We constructed a series of envelope mutants to characterize the SU peptide of SNV. Point mutations were introduced throughout SU in regions that are conserved among all retroviruses belonging to the same receptor interference group. The biological and biochemical properties of these mutants were analyzed. All mutants were transported efficiently to the cell surface. Almost all mutations in the amino-terminal one-third caused a conformational change of the envelope and a significant drop in infectivity and abolished the ability to confer superinfection interference. Similar observations were made with only two of seven mutants with mutations in the middle of SU. Four mutations in this region had little or no effect on biological activity. One mutant envelope protein (Asp to Arg at position 192) was processed normally but showed little infectivity and had no ability to confer superinfection interference. A detailed mutational analysis suggested that this amino acid forms a hydrogen bond to its cellular receptor. Mutations within the carboxy-terminal part of SU had very little or no effect on biological function. Aberrantly processed envelope proteins were proteolytically cleaved at a new point upstream of and differing in sequence from the conserved retroviral SU/TM cleavage site. Surprisingly, these mutants still retained some infectivity (0.01 to 1% of that of the wild type). Our data indicate that the envelope of SNV behaves in a manner very different from that of the envelopes of other studied retroviruses.