The primary structure of the receptor for platelet-derived growth factor (PDGF), determined by means of cloning a cDNA that encodes the murine pre-PDGF receptor, is closely related to that of the v-kit oncogene product and the receptor for macrophage colony stimulating factor (CSF-1). Common structural features include the presence of long sequences that interrupt the tyrosine-specific protein kinase domains of each molecule. The PDGF and CSF-1 receptors also share a characteristic distribution of extracellular cysteine residues. Ubiquitin is covalently bound to the purified PDGF receptor, the human gene for which is on chromosome 5.

Based on nucleotide sequence analysis of the hemagglutinin (HA) gene from the virulent and avirulent A/chicken/Pennsylvania/83 influeza viruses, it was previously postulated that acquisition of virulence was associated with a point mutation that resulted in loss of a glycosylation site. Since there are two potential glycosylation sites in this region of the HA molecule and since all Asn-Xaa-Thr/Ser sequences in the HAs of different strains are not necessarily glycosylated, the question remained open as to whether either one of these sites was glycosylated. We now provide direct evidence that a site-specific glycosylation affects cleavage of the influenza virus HA and thus virulence. We have identified the glycosylation sites on the HAl subunit from the virulent and avirulent strains by direct structural analysis of the isolated proteins. Our results show that the only difference in glycosylation between the HAls of the virulent and avirulent strains is the lack of an asparagine-linked carbohydrate on the virulent HAl polypeptide at residue 11. Further, we show that the HAls of both the avirulent and virulent viruses are not glycosylated at one potential site, while all other sites contain carbohydrate. Amino acid sequence analysis of the HAl of an avirulent revertant of the virulent strain confirmed these findings.In April 1983, an avirulent (avir) influenza virus, A/chicken/Pennsylvania/1/83 (Chick/Penn) (HSN2), appeared in chickens of eastern Pennsylvania (1). By October 1983, this virus had become virulent (vir), causing up to 80% mortality in domestic poultry (1, 2). Gene reassortment studies established that the difference between the viruses was a mutation of the hemagglutinin (HA) (3), a major surface glycoprotein responsible for viral attachment to, and penetration of, host cells. Nucleotide sequencing analysis suggested that the critical mutation eliminated a possible site for the attachment of an N-linked oligosaccharide at Asn-11 of HAl of the avirulent virus by altering a sequence for glycosylation. Preliminary evidence based on molecular weight differences in HAl and growth in the presence of tunicamycin suggested loss of a carbohydrate (4). However, since not all Asn-XaaSer/Thr sites are glycosylated in different influenza virus HAs, we do not know how many of the potential glycosylation sites contain carbohydrate, and since all of the influenza virus HAs studied to date contain a carbohydrate at this location in the HA molecule (5, 6), the question of glycosylation at this site remained unresolved.The HA protein of influenza viruses is post-translationally modified by cleavage at a connecting peptide region into subunits HA1 and HA2 (7). This cleavage is a prerequisite for viral infectivity (8), and a correlation has been drawn between HA cleavage and virus production in tissue culture and the virulence of the strain (9-11). Although exogenously added trypsin can cleave the HA and permit infectivity in vitro, the mechanism by which HA is processed and cleaved in vivo is unknown.To determin...

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