For such a tiny entity, HIV-1 has a very complicated life. Its study has generated numerous drugs that interfere with key life events, and have brought it under control in most developed areas of the world. But there is a lot left to learn. While its fusion with target cell membranes and eventual replication have received the most study, less is known about what happens in between.
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| The layers of HIV. Outside is a membrane, which features proteins like gp120 that binds to T-cell receptors. |
The virus has several layers. On the outside is a membrane coat, designed to protect the virus in moist environments, but more imporantly to expose surface proteins that seek out T-cells specifically, and then to fuse with their membranes, thus entering them. Inside the lipid membrane and its supporting proteins is the capsid, a protein coat that protects the HIV genome from attack by internal T-cell defenses like RNases, helicases, RNA sensing proteins, interferons, and their various downstream effects. This capsid finally lets go if its cargo at the nucleus, where the viral genome, now transformed into double stranded DNA, integrates itself into the cell's genome.
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| Electron micrograph of HIV particles. The capsid sits inside the membrane coat, all of which is about 120 nm in diameter. |
Innermost is the genome, composed of RNA, not DNA. Packaged alongside it are key enzymes integrase, RNase H, and reverse transcriptase. Once HIV enters the cell, and meets the rich mother-lode of ATP and other nucleotides, including deoxy-ribonucleotides that are the building blocks of DNA, the reverse transcriptase can begin its work. It copies the genome in a complex sequence of starts within the linear RNA, hops to the other end of the RNA, continuation, and finally termination. All the while, the RNase digests the original RNA genome, in a remarkable process of self-transmutation. It is thought that synthesis of the complementary DNA strand only begins after the virus rides the cell's light rail system (microtubules) to the nucleus, where it leaves its capsid behind.
Some recent papers shed a bit of light on the nature of the HIV capsid, which is more interesting than previously understood. It was not previously clear whether the capsid uncoats at the membrane during original entry, or only at the nucleus, or even inside the nucleus. But one paper shows quite definitively that the capsid remains intact through the first strand of DNA synthesis, thus through most or all of the virus's trip from the outside membrane the nucleus. This work required rather difficult assays for capsid integrity, judged by the inclusion of fluorescent protein GFP into the viral genome. Most of this protein escaped during viral fusion with the membrane, since most of the free volume of the virus is outside the capsid. But a small portion remained visible as long as the capsid remained intact.
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| One hexamer of the capsid protein CA. At the center is pore, accommodating a very small molecule of ATP, in yellow/orange. The surrounding blue parts of the protein are positively charged, ideally suited to attract nucleotides and the IP6 stabilizing molecule. |
Another paper took a closer look at the stability and composition of the capsid. It is known to be composed of roughly 1500 copies of the viral CA protein, in multimeric (hexameric and pentameric) rosettes. The structure above shows that at the center of these rosettes is a small pore, big enough to let in ATP- suggesting clearly that the deoxynucleotides needed for reverse transcription can enter even while the capsid is intact, providing the virus with the best of both worlds- protection from the cell's various specific antiviral defenses, but also food for its replication. In the lab, these capsids have been remarkably unstable, however, leading some to believe that the virus uncoats soon after it enters the cell. But these authors find that a chemical that is obscure, but common in cells, IP6, is shaped just right, and negatively charged just right, to sit in these rosette pores and stabilize the whole structure, extending the lifespan of capsids in the test tube to hours instead of minutes.
Two mysteries remain- first is how deoxyribonucleotides get in if IP6 is blocking the pores and stabilizing the structure. The likely answer is simply that these small molecules are not plugs. They have more stochastic behavior, jumping on and off frequently enough to allow other small molecules occasional access. The second question is what finally causes the capsid to unravel at the nucleus and release its now-DNA genome to join that of the cell. It is thought that these capsids are too large to go through the nuclear pore, so they may dock in some fashion and only transmit their contents, probably using special signals that mimick one of the many other proteins and molecules that are regularly imported into the nucleus. That process is currently unknown, and may be another avenue of viral inhibition and drug development.
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