The viral cores that are injected into target cells by HIV-1 possess a complex proteome that may, through characterization, offer insight into producer-specific mechanisms of viral infection and give rise to innovative therapeutic options for treatment.
Recently, Santos et al.1 analyzed the viral cores of HIV-1 NL4-3 virions assembled in T lymphocytes, as well as in models of both macrophages and monocytes. The researchers were able to isolate 202 cellular proteins, 42 of which were present in virions assembled in T cells and both types of myeloid cells. This represents 20% of the detected proteins and indicates that these specific proteins may play a role in viral replication.
In order to attain these results, Santos et al. used ultra-speed centrifugation coupled with a sucrose density gradient and detergent layer to isolate mostly mature virion cores. This process extricates the cores from the extracellular structures and glycoproteins that make up the viral envelope. The researchers then used liquid chromatography mass spectrometry with an LTQ Orbitrap XL and spectral analysis with Proteome Discoverer 1.2 software (Thermo Scientific) to isolate the initial 202 cellular proteins. These proteins were classified into 31 categories, according to function and location within the cellular structure. In order of prevalence, the majority of the identified proteins were found to be RNA-binding proteins, cytoskeleton components and regulators, chaperones, DNA-binding proteins, vesicular transporters, and ubiquity-protease components. Of the 42 proteins common to all three preparations, the majority of the proteins were chaperones, cytoskeleton components, and proteins related to vesicular transport. These specific protein categories have been previously demonstrated to play a role in viral protein folding and HIV virion assembly.
Santos et al. found that the type of cell from which the virion core was derived impacts the proteomic makeup of the virus. The 42 proteins that were common to all three preparations represent 34% of the total proteins found in the cores of viruses assembled in T cells and 38% and 47% of the proteins found in the viral cores assembled in models of macrophages and monocytes, respectively. In comparison to the virions assembled in T cells, macrophage-derived viral cores revealed 12% fewer host proteins and monocyte-derived viral cores had 28% fewer host proteins. It is likely that this differential expression of host proteins is a result of more efficient virion assembly in T cells. RNA and DNA helicases were also found to be packaged more efficiently in virions derived from T cells.
The researchers compared the quantity seven specific common proteins by western blot analysis. These proteins were DHX9 (RHA), SNRNP200 (HELIC2), MCM5 (CDC46), XRCC5 (Ku80), Beta tubulin, RUVBL1 (Pontin52), and RUVBL2 (Reptin 52). Other than beta tubulin, and RUVBL1 and RUVBL2 helicases, which did not reveal differential expression, the other four proteins were found to exhibit increased abundance in virions assembled in T cells. Santos et al. indicate that this points to the fact that, in T cells, viral assembly takes place in the plasma membrane.
While previous work has been done in the area of cellular protein profiling, this study represents a specific characterization of host cell proteins in HIV-1 virion cores. The finding that viral core protein profiles vary based on the producer cell and is essentially non-random may aid researchers in further understanding the mechanics of HIV infection and give direction in the identification of targets for antiretroviral therapy.
1. Santos, S., et al. (2012) ‘Virus-producing cells determine the host protein profiles of HIV-1 virion cores‘, Retrovirology, 9 (65), (pp. 2-28)
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