STRUCTURE AND FUNCTION OF A MEMBRANE ANCHOR-LESS FORM OF THE HEMAGGLUTININ-NEURAMINIDASE GLYCOPROTEIN OF NEWCASTLE-DISEASE VIRUS

The hemagglutinin-neuraminidase (HN) glycoprotein of paramyxoviruses i s anchored in the virion membrane near its amino terminus, protruding from the virion surface to mediate attachment to cellular receptors. S olubilization of HN spikes can be achieved by treatment of virions wit h detergent and high salt concentrations. When the solubilized HN prot ein from the Australia-Victoria (AV) isolate of the virus is incubated at 37-degrees-C, a chymotrypsin-sensitive site between residues 112 a nd 113 is exposed. A chymotrypsin-cleaved soluble form of the protein, named CT-HN, has been prepared using this approach. It is membrane an chor-less, due to removal of a 14-kDa fragment from the NH2 terminus o f HN. It retains all potential glycosylation sites and cysteines prese nt in the ectodomain of the native protein. It migrates in nonreducing gels and sediments in sucrose gradients at the rate expected for homo dimeric HN. The latter is also consistent with our demonstration by si te-directed mutagenesis that cysteine residues at positions 6 and 123, respectively, mediate disulfide-linked homotetramer and homodimer for mation. CT-HN retains almost total antigenicity, suggesting that it is conformationally very similar to the intact molecule, as well as rece ptor recognition function and, at low pH, neuraminidase activity. It s hould prove to be a useful tool for further studies of the structure a nd function of this important viral glycoprotein.