Assembly, processing, and infectivity of human immunodeficiency virus type 1 gag mutants

Genetic analysis of the major homology region of the Rous sarcoma virus Gag protein

The mature cores of all retroviruses contain a major structural protein known as the CA (capsid) protein. Although it appears to form a shell around the ribonucleoprotein complex that contains the viral RNA, its function in viral replication is largely unknown. Little sequence similarity exists between the CA proteins of different retroviruses, except for a region of about 20 amino acids termed the major homology region (MHR). To examine the role of the CA protein in particle assembly and release, mutants of Rous sarcoma virus were created in which segments of CA were deleted or single conserved residues in the MHR were altered. The ability of the deletion mutants to release particles at rates similar to the wild-type protein demonstrated that the CA domain of Gag is not an essential component of the minimal budding machinery. Certain point mutations in the MHR region did block assembly and release in certain cell types, presumably by perturbing the global structure of the Gag precursor. Another group of MHR substitutions produced noninfectious or poorly infectious particles that were normal in their content of gag and pol gene products and viral RNA. The mutants were capable of initiating reverse transcription in vitro; however, the association of CA protein with the core was compromised, as indicated by its sensitivity to extraction with nonionic detergent. Prominent blebs on the virion envelope also indicated a disturbance at the membrane. Finally, an anti-peptide serum directed against MHR was found to react with the uncleaved Gag protein but not with mature CA, suggesting that MHR undergoes a dynamic rearrangement upon liberation from the polyprotein. We conclude that the MHR is involved in the very late steps in maturation of the virion (i.e., ones that occur after budding is initiated) and is essential for proper function of the core upon entry into a new host cell.

Use of a novel human immunodeficiency virus type 1 reporter virus expressing human placental alkaline phosphatase to detect an alternative viral receptor

We report here on the construction and use of a novel human immunodeficiency virus (HIV) type 1 reporter vector, HIV-AP, that encodes human placental alkaline phosphatase. Upon staining with chromogenic alkaline phosphatase substrates 24 to 36 h postinfection, cells infected with HIV-AP develop an intense purple color and can then be counted under a dissecting microscope. Alternatively, HIV-AP infectivity can be quantitated and infected cells can be sorted by a fluorescence-activated cell sorter after staining with a fluorescent alkaline phosphatase substrate. The assay is rapid and accurate, has very low background in a variety of cell lines and primary cells, and is not restricted to use in human cells. Infectious HIV-AP can be pseudotyped by various HIV or murine leukemia virus envelope glycoproteins. Using this virus, we have addressed the long-standing question of CD4-independent infection of cells by HIV. Our results confirm the presence on a human osteosarcoma cell line of an alternative receptor for HIV infection that functions with an efficiency approximately 1/20 that of CD4.

Rescue of human immunodeficiency virus type 1 matrix protein mutants by envelope glycoproteins with short cytoplasmic domains

The matrix (MA) protein of human immunodeficiency virus type 1 (HIV-1) forms the outer protein shell directly underneath the lipid envelope of the virion. The MA protein has a key role in different aspects of virus assembly, including the incorporation of the HIV-1 Env protein complex, which contains a transmembrane glycoprotein with an unusually long cytoplasmic tail. In this study, we compared the abilities of HIV-1 MA mutants to incorporate Env protein complexes with long and short cytoplasmic tails. While the mutant particles failed to incorporate the authentic HIV-1 Env protein complex, they retained the ability to efficiently and functionally incorporate the amphotropic murine leukemia virus Env protein complex, which has a short cytoplasmic tail. Moreover, incorporation of the autologous Env protein complex could be restored by a second-site mutation that resulted in the truncation of the cytoplasmic tail of the HIV-1 transmembrane glycoprotein. Remarkably, the second-site mutation also restored the ability of MA mutants to replicate in MT-4 cells. These results imply that the long cytoplasmic tail of the transmembrane glycoprotein is responsible for the exclusion of the HIV-1 Env protein complex from MA mutant particles.

Role of the basic domain of human immunodeficiency virus type 1 matrix in macrophage infection

The matrix domain of the human immunodeficiency virus type 1 (HIV-1) Gag protein contains a highly basic region near its amino terminus. It has been proposed that this basic domain, in conjunction with the HIV-1 accessory protein Vpr, is responsible for the localization of the HIV-1 preintegration complex to the nucleus in nondividing cells. It has also been postulated that the matrix basic domain assists in the targeting of the HIV-1 Gag precursor Pr55Gag to the plasma membrane during virus assembly. To evaluate the role of this highly basic sequence during infection of primary human monocyte-derived macrophages, single- and double-amino-acid-substitution mutations were introduced, and the effects on virus particle production, Gag protein processing, envelope glycoprotein incorporation into virus particles, and virus infectivity in the CEM(12D-7) T-cell line, peripheral blood mononuclear cells, and primary human monocyte-derived macrophages were analyzed. Although modest effects on virus particle production were observed with some of the mutants, none abolished infectivity in primary human monocyte-derived macrophages. In contrast with previously reported studies involving some of the same matrix basic domain mutants, infectivity in monocyte-derived macrophages was retained even when combined with a vpr mutation.

Sequence requirements for encapsidation of deletion mutants and chimeras of human immunodeficiency virus type 1 Gag precursor into retrovirus-like particles

Interacting domains in human immunodeficiency virus type 1 (HIV-1) Gag precursor (Pr55gag) expressed in recombinant baculovirus-infected cells were investigated by three different methods: (i) trans rescue and coencapsidation of C-terminal deletion (amber) Gag mutants and Gag chimeras into retrovirus-like particles in complementation experiments with HIV-1 wild-type (WT) Pr55gag, (ii) Gag-Gag interactions in vitro in Gag ligand affinity blotting assays, and (iii) quantitative immunoelectron microscopy of retrovirus-like Gag particles, using a panel of monoclonal antibodies to probe the epitope accessibility of encapsidated HIV-1 WT Pr55gag. Four discrete regions, within residues 210 to 241, 277 to 306 (major homology region), and 307 to 333 in the capsid (CA) protein and residues 358 to 374 at the CA-spacer peptide 2 (sp2) junction, were found to have a significant influence on Gag trans-packaging efficiency. A fifth region, within residues 375 to 426, overlapping the sp2-nucleocapsid (NC) protein junction and most of the NC, seemed to be essential for stable inter-Gag binding in vitro. The coincidence of the two regions from 358 to 374 and 375 to 426 with an immunologically silent domain in WT Gag particles suggested that they could participate in direct Gag interactions.

Linker insertion mutations in the human immunodeficiency virus type 1 gag gene: effects on virion particle assembly, release, and infectivity

The phenotypes of a series of mutant human immunodeficiency virus type 1 proviruses with linker insertion and deletion mutations within the gag coding region were characterized. These mutants were tested for their ability to make and release viral particles in COS7 cells and for their viability in vivo. Of the 12 mutant proviruses, 4 did not make extracellular virion particles when transfected into COS7 cells. All four of these mutants had mutations in the C-terminal domain of CA. These mutants appeared to have defects both in the ability to accumulate high-molecular-weight intracellular structures containing Gag and Pol products and in the ability to release virion particles. Seven of the mutant proviruses retained the ability to make, release, and process virion particles from COS7 cells. These particles contained the Env glycoprotein, viral genomic RNA, and the mature products of the Gag and Gag-Pol polyproteins, yet they were noninfectious or poorly infectious. The defect in these mutants appears to be in one of the early steps of the viral life cycle. Thus, multiple regions throughout Gag appear to be important in mediating the early steps of the viral life cycle.

Human immunodeficiency virus type 1 MA deletion mutants expressed in baculovirus-infected cells: cis and trans effects on the Gag precursor assembly pathway

The role of the matrix protein (MA) of human immunodeficiency virus type 1 in intracellular transport, assembly, and extracellular release of Gag polyprotein precursor (Pr55gag) was investigated by deletion mutagenesis of the MA domain of recombinant Gag precursor expressed in baculovirus-infected cells. In addition, three carboxy-terminally truncated forms of the Gag precursor, representing mainly the MA, were constructed. One corresponded to an MA with a deletion of its last 12 residues (amb120), while the others corresponded to the entire MA with an additional sequence from the N-terminal portion of the CA (amb143 and och180). Deletions within the MA central region (residues 41 to 78) appeared to be detrimental to Gag particle assembly and budding from the plasma membrane. A slightly narrower domain, between amino acids 41 and 68, was found to be critical for soluble Gag secretion. Mutations which totally or partially deleted one or the other of the two polybasic signals altered the transport of N-myristylated Gag precursor to the plasma membrane. In coexpression with wild-type Gag precursor, a discrete trans-dominant negative effect on wild-type Gag particle assembly and release was observed with deletion mutants located in the central MA region (residues 41 to 78). A more significant negative effect was obtained with the two recombinant proteins of amb120 and och180, which redirected the Gag particle assembly pathway from the plasma membrane compartment to intracellular vesicles (amb120) and to the nuclear compartment (och180).

Functional domains of the capsid protein of human immunodeficiency virus type 1

A series of deletions was introduced into the CA domain of the human immunodeficiency virus type 1 Gag polyprotein to examine its role in virus particle and core formation. The mutations resulted in two phenotypes, indicating the existence of two functionally distinct regions within the CA domain. Deletions within a conserved stretch of 20 amino acids referred to as the major homology region (MHR) and deletions C terminal to this region blocked virus replication and significantly reduced the ability to form viral particles. Deletions N terminal to the MHR also prevented virus replication, but the mutants retained the ability to assemble and release viral particles with the same efficiency as the wild-type virus. The mutant particles contained circular rather than cone-shaped cores, and while they were of a density similar to that of wild-type particles, they were more heterogeneous in size. These results indicate that CA domain sequences N terminal to the MHR are essential for the morphogenesis of the mature cone-shaped core.

Specificity and sequence requirements for interactions between various retroviral Gag proteins

We previously established a genetic assay for retroviral Gag polyprotein multimerization (J. Luban, K. B. Alin, K. L. Bossolt, T. Humaran, and S. P. Goff, J. Virol. 66:5157-5160, 1992). Here we use this assay to demonstrate homomeric interactions between Gag polyproteins encoded by six different retroviruses. Of the Gag polyproteins tested, only those encoded by closely related retroviruses formed heteromultimers. To determine the primary sequence requirements for human immunodeficiency virus type 1 Gag polyprotein multimerization, we studied the effects on multimerization of deletion and linker insertion mutations. Sequences necessary for this process were located between the C-terminal one-third of the capsid domain and the C terminus of the nucleocapsid domain.

Single amino acid changes in the human immunodeficiency virus type 1 matrix protein block virus particle production

The matrix protein of human immunodeficiency virus type 1 is encoded by the amino-terminal portion of the Gag precursor and is postulated to be involved in a variety of functions in the virus life cycle. To define domains and specific amino acid residues of the matrix protein that are involved in virus particle assembly, we introduced 35 amino acid substitution mutations in the human immunodeficiency virus type 1 matrix protein. Using reverse transcriptase and radioimmunoprecipitation analyses and transmission electron microscopy, we assessed the mutants for their ability to form virus particles and to function in the infection process. This study has identified several domains of the matrix protein in which single amino acid substitutions dramatically reduce the efficiency of virus particle production. These domains include the six amino-terminal residues of matrix, the region of matrix between amino acids 55 and 59, and the region between amino acids 84 and 95. Single amino acid substitutions in one of these domains (between matrix amino acids 84 and 88) result in a redirection of the majority of virus particle formation to sites within cytoplasmic vacuoles.

Phenotypic characterization of insertion mutants of the human immunodeficiency virus type 1 Gag precursor expressed in recombinant baculovirus-infected cells

A panel of 28 insertion mutants of the human immunodeficiency virus type 1 (HIV-1) Gag precursor (Pr55Gag) was constructed by linker-insertion mutagenesis and expressed in recombinant baculovirus-infected insect cells. One set of 14 mutants carried the normal N-myristylation signal; the other set constituted their non-N-myristylated counterparts. The mutants were characterized with respect to (i) assembly and extracellular release of membrane-enveloped budding Gag particles, (ii) intracellular assembly and nuclear transport of Gag cores, (iii) specific processing of Pr55Gag by HIV-1 protease in vivo, and (iv) binding of Pr55Gag to an HIV-1 genomic RNA probe in Northwestern blotting. Insertions within the region between amino acid residues 209 and 334 in the CA domain appeared to be the most detrimental to Gag particle assembly and release of Gag into the external medium, whereas a narrower window, between residues 209 and 241, was found to be critical for secretion of soluble Pr55Gag. Differences in Pr55Gag processing in vivo and RNA binding in vitro between N-myristylated and non-N-myristylated Gag mutants suggested a major conformational role for the myristylated N terminus of Gag precursor. In coinfection experiments using wild-type Gag- and mutant Gag-expressing recombinants, a transdominant negative effect on Gag particle assembly and release was observed for insertions located in two separate domains, the matrix and nucleocapsid.

Conditional infectivity of a human immunodeficiency virus matrix domain deletion mutant

We constructed a human immunodeficiency virus (HIV) matrix (MA) deletion mutant by deletion of about 80% of the HIV type 1 Gag MA domain but retaining myristylation and proteolytic processing signals. The effects of this deletion matrix (dl.MA) mutant on HIV particle assembly, processing, and infectivity were analyzed. Surprisingly, the dl.MA mutant still could assemble and process virus particles, had a wild-type (wt) retrovirus particle density, and possessed wt reverse transcriptase activity. RNase protection experiments showed that dl.MA mutant particles preferentially packaged viral genomic RNA. When both mutant and wt particles were pseudotyped with an amphotropic murine leukemia virus envelope protein, mutant infectivity was about 10% of wt level. In contrast, infectivity of the dl.MA mutant was 1,000-fold less than that of wild-type when mutant and wt particles were pseudotyped with the HIV envelope protein. Protein analyses of pseudotyped virions indicated that there were no major differences between mutant and wt viruses in the efficiency of amphotropic murine leukemia virus envelope protein incorporation. In contrast, there was a reduction in the amount of mutant particle-associated HIV envelope protein gp120. Our results suggest that an intact HIV matrix domain is not absolutely required for reverse transcription, nuclear localization, or integration but is necessary for appropriate HIV envelope protein function.