The complete nucleotide sequence of two human T-cell leukaemia type III (HTLV-III) proviral DNAs each have four long open reading frames, the first two corresponding to the gag and pol genes. The fourth open reading frame encodes two functional polypeptides, a large precursor of the major envelope glycoprotein and a smaller protein derived from the 3'-terminus long open reading frame analogous to the long open reading frame (lor) product of HTLV-I and -II.

Some cases of hereditary nonpolyposis colorectal cancer (HNPCC) are due to alterations in a mutS-related mismatch repair gene. A search of a large database of expressed sequence tags derived from random complementary DNA clones revealed three additional human mismatch repair genes, all related to the bacterial mutL gene. One of these genes (hMLH1) resides on chromosome 3p21, within 1 centimorgan of markers previously linked to cancer susceptibility in HNPCC kindreds. Mutations of hMLH1 that would disrupt the gene product were identified in such kindreds, demonstrating that this gene is responsible for the disease. These results suggest that defects in any of several mismatch repair genes can cause HNPCC.

The effects on human immunodeficiency virus type 1 virion morphogenesis and on virus replication of mutations that affect posttranslational processing of the capsid precursor protein are described. A change in the glycine residue at position two from the N terminus abolishes the myristoylation of the precursor proteins and also prevents virus particle release. Mutations in the viral protease gene abolish proteolytic cleavage of the capsid precursor but do not prevent the formation and budding of virion particles of immature appearance. Mutations that alter the sequence of the sites normally used for cleavage of the major capsid protein p24 from the capsid precursor alter virion morphogenesis and prevent virus replication.Electron microscopy of infected cells reveals that particle formation of the human immunodeficiency virus type 1 (HIV-1) begins when an electron-dense structure assembles in patches under the cell membrane (1-3). As the structure assumes a more spherical shape, a portion of the cell membrane pinches off to surround the budding structure. During the course of budding, no clear central core of the virus particle is apparent. After release from the cell, a coneshaped inner core particle is formed (2).What features of the capsid proteins and their precursor polypeptides determine this maturation process? The precursor of the virion capsid proteins is a 55-kDa polyprotein (p55) encoded by the gag region of the viral genome (4). A 160-kDa polyprotein, which is made as a consequence of a -1 frameshift between the gag and pol frames (5), contains precursor forms of the enzymes necessary for virus replication in addition to the capsid proteins. The N terminus ofboth the 55-kDa and 160-kDa proteins is myristoylated (6). Addition of myristic acid to proteins typically occurs cotranslationally (7) and is specified by the sequence ofthe N terminus of the protein (8).Cleavage of both the capsid precursor to yield the mature p17, p24, p9, and p6 capsid proteins as well as the precursor of the replication activities is at least in part specified by the viral protease (9, 10). The viral protease is encoded by the N terminus of the pol reading frame. The protease is reported to be active only as a dimer (11) and has been demonstrated to exhibit a preference for certain natural cleavage sites over others (12).The effects of mutations that prevent the capsid protein myristoylation and the protease activity of HIV-1 on virus assembly and virion morphology have not been reported. The experiments reported here reveal the effects on capsid protein processing, virion morphogenesis, and virus replication of a series of mutations that prevent myristoylation, inactivate the viral protease, or alter the sequence of the cleavage sites. MATERIALS AND METHODSConstruction of Mutants. A 1.3-kilobase (kb) Sac I-Apa I gag fragment from pHXB-SV (13) was subcloned into pBluescriptSK(+) (Stratagene) to generate pSK'gag as a target plasmid for the mutagenesis of the myristoylation site and the N-terminal p24 cleavage site. A 4...

The envelope of the human immunodeficiency virus type 1 (HIV-1) plays a central role in the process of virus entry into the host cell and in the cytopathicity of the virus for lymphocytes bearing the CD4 molecule. Mutations that affect the ability of the envelope glycoprotein to form syncytia in CD4+ cells can be divided into five groups: those that decrease the binding of the envelope protein to the CD4 molecule, those that prevent a post-binding fusion reaction, those that disrupt the anchorage of the envelope glycoprotein in the membrane, those that affect the association of the two subunits of the envelope glycoprotein, and those that affect post-translational proteolytic processing of the envelope precursor protein. These findings provide a functional model of the HIV envelope glycoprotein.

Hereditary nonpolyposis colorectal cancer (HNPCC) is one of man's commonest hereditary diseases. Several studies have implicated a defect in DNA mismatch repair in the pathogenesis of this disease. In particular, hMSH2 and hMLH1 homologues of the bacterial DNA mismatch repair genes mutS and mutL, respectively, were shown to be mutated in a subset of HNPCC cases. Here we report the nucleotide sequence, chromosome localization and mutational analysis of hPMS1 and hPMS2, two additional homologues of the prokaryotic mutL gene. Both hPMS1 and hPMS2 were found to be mutated in the germline of HNPCC patients. This doubles the number of genes implicated in HNPCC and may help explain the relatively high incidence of this disease.

Mutations in sequences at the C terminus of the capsid precursor protein of human immunodeficiency virus type 1 that affect the viral p6 protein prevent release of budded virus particles from the cell surface. The experiments reported here define an important step in the life cycle of the virus, the release of the budded particle from a tether that binds the assembled particle to the cell surface. Inhibition of the release of the viral capsid proteins by interferon a indicates that this step of virus maturation may be sensitive to inhibition by antiviral drugs.

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Acquired immune deficiency syndrome (AIDS) is characterized by marked depletion of the T4+ helper subset of T cells. The aetiological agent of the disease, the human T-lymphotropic virus type III (HTLV-III)/lymphadenopathy-associated virus (LAV), specifically kills T4+ cells in vitro. Part of this specificity for the T4+ population residues in the relative efficiency with which HTLV-III infects these cells, as a result of a specific interaction between the T4 molecule and the virus envelope glycoprotein. In addition, the cytotoxic consequences of HTLV-III replication are dependent on cell type, as certain lymphoid and myeloid cells can be productively infected without notable cytopathic effect. Here we investigate the basis for the specific cytotoxicity of the virus, and report that high-level expression of the HTLV-III envelope gene induces syncytia and concomitant cell death in T4+ cell lines but not in a B-lymphocyte line. Syncytium formation depends on the interaction of envelope-expressing cells with neighbouring cells bearing surface T4 molecules. These results explain, at least in part, the specific cytopathic effect of HTLV-III infections.