We isolated overlapping cDNA clones corresponding to the major MET protooncogene transcript. The cDNA nucleotide sequence contained an open reading frame of 1408 amino acids with features characteristic of the tyrosine kinase family of growth factor receptors. These features include a putative 24-amino acid signal peptide and a candidate, hybrophobic, membrane-spanning segment of 23 amino acids, which defines an extracellular domain of 926 amino acids that could serve as a ligand-binding domain. A putative intracellular domain 435 amino acids long shows high homology with the SRC family of tyrosine kinases and within the kinase domain is most homologous with the human insulin receptor (44%) and v-abl (41%). Despite these similarities, however, we found no apparent sequence homology to other growth factor receptors in the putative ligand-binding domain. We conclude from these results that the MET protooncogene is a cell-surface receptor for an as-yet-unknown ligand.

The envelope (env) and structural (gag) gene products of human T-cell leukemia (lymphotropic) virus type III were identified by immunoaffinity chromatography, immunoprecipitation, and two-dimensional oligopeptide mapping methods. The env gene specifies a glycosylated polypeptide with a molecular weight of 160,000 (gp160) that is processed to gp120 and smaller gene products. The gag gene specifies two polypeptides of 70,000 and 55,000 molecular weight (p70 and p55), both of which contain p24, the major structural protein of the mature virion. The techniques in this study can be used to define the extent of variability of the env gene product among different virus isolates and may identify the nature and patterns of the humoral immune response that lead to an immunologically protected state.

A simple, rapid, quantitative syncytium-forming microassay for the detection of human immunodeficiency virus (HIV-I) isolates is described. A virus-syncytial sensitive clone of CEM cells (CEM-SS) was identified and made adherent to flat bottom 96-well microtiter dishes. Following the addition of virus, these cells develop easily quantifiable, adherent syncytia on a background of confluent, normal CEM-SS monolayer in 4 to 6 days. One-hit kinetics for syncytia formation were obtained at various multiplicities of infection. Syncytia are associated with complete virion production and cytoplasmic localization of the p24 core protein (detected by immunofluorescence). Total infectious virus can be accurately determined in this assay; these results showed a close correlation with p24 and gp120 induction when microtiter well supernatants were passed to fresh cells and evaluated by competitive radioimmunoassay. Studies of p24 antigen induction at and beyond the end point of syncytia formation indicate that there are no detectable nonsyncytial variants in standard HIV-I stocks. Six divergent HIV-I isolates (HTLV-IIIB, -RFII, -MN, -RUTZ, -CC, and LAV-1), as well as HTLV-IIIB and LAV-1 reisolated from persistently infected chimpanzees, produce quantifiable syncytia which vary slightly in their developmental morphology. Accurate neutralization titers are readily obtained from easily constructed multiplicity curves derived from serial dilutions of test sera. Inherent within this system is a flexible method for studying various kinetics of antibody/virus interactions, as well as blocking and interference studies with any candidate antiviral compounds.

Peripheral blood leukocytes and saliva from 20 individuals, including four with the acquired immune deficiency syndrome (AIDS), ten with AIDS-related complex (ARC), and six healthy homosexual males at risk for AIDS, were compared as sources of transmissible human T-cell leukemia (lymphotropic) virus type III (HTLV-III), the virus found to be the etiologic agent of AIDS. All of the AIDS and ARC patients and four of the six healthy homosexuals had evidence of prior exposure to HTLV-III as indicated by seropositivity for antibody to HTLV-III structural proteins. Infectious virus was isolated from the peripheral blood of one of the AIDS patients, four of the ARC patients, and two of the healthy homosexual males, consistent with previous reports. HTLV-III was also isolated from the saliva of four of the ARC patients and four of the healthy homosexuals. Virus was also observed by electron microscopy in material prepared by centrifugation of the saliva of one AIDS patient. Although AIDS does not appear to be transmitted by casual contact, the possibility that HTLV-III can be transmitted by saliva should be considered.