Incorporation of sulfate into influenza virus glycoproteins

The glycoproteins of the influenza virion are labeled with [³⁵S]O₄, whereas the nonglycosylated virion polypeptides are unlabeled. Virions grown in the absence of serum contain sulfate label incorporated into both the hemagglutinin (HA) and neuraminidase (NA) polypeptides; in the presence of serum, the HA₁ and HA₂ cleavage products also both contain sulfate label, with HA, containing three to five times more label than HA₂. In addition to the glycoproteins of the virion, sulfate is incorporated into a component with an electrophoretic mobility lower than that of any of the virion proteins in sodium dodecyl sulfate-polyacrylamide gels. Unlike the virion glycoproteins, this component is also labeled when virions are grown in cells preincubated with [³⁵S]O₄ prior to infection, and it appears to be a host cell-derived sulfated mucopolysaccharide. Labeling of the glycoproteins and the mucopolysaccharide with [³⁵S]O₄ was observed in virions grown in both MDBK and BHK21-F cells. It was estimated that at least ∼0.5 mole of sulfate is incorporated per mole of HA polypeptide in virions grown in MDBK cells. Treatment of purified virions with Triton X-100, which solubilizes the glycoproteins, does not solubilize the sulfated polysaccharide or the nonglycosylated polypeptides. After acid hydrolysis of the isolated glycoproteins, the sulfate label is precipitable with BaCl₂, indicating that it is incorporated as sulfate per se rather than a metabolic product. Treatment with the protease bromelain, which produces spikeless particles lacking the glycoproteins, also removes the polysaccharide, indicating that it is located on the external surface of the viral envelope. The polysaccharide is also removed by mild trypsin treatment, under conditions where HA₁ and HA₂ remain intact.