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After a virus (one of the varieties which infects the cell via injection and not endocytosis) injects its genetic material into the host cell, what happens to its protein coat? I would guess that it just falls off. However, since it is now outside of a cell, there are no protein-digesting enzymes -- or are there?

What happens to it now? I'm guessing it doesn't just float around for all eternity.

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Interesting question and actually I haven't found a real answer here yet. The phages seem to bind rather hard, since the need to be blended (subjected to relatively high forces) to shear off. This was used in the Hershey-Chase-Experiment to find out what the genetic material of the cell is. It doesn't matter though, since most of the cells which are infected get destroyed anyway. If not, they are most likely taken up and broken down inside the cell, as it happens with other membrane proteins. This is only valid for viruses which inject their genetic material into the host cell, while the capsid stays outside. This happens mostly for Bacteriophages.

There are a few other possibilities for viruses to enter cells where this is no problem:

Viruses which have a membrane envelope can enter the cell via membrane fusion. To make the fusion happen, either the membranes have to get very close, or the virus first binds receptors on the cell surface and then fuses its membrane with the cell. This causes the interior of the virus to be released into the cell. The picture below shows the fusion (from the Wikipedia article on membrane fusion). An interesting article on this topic is: "Membrane fusion."

enter image description here

Another possibility for a virus to enter a host cell is via endocytosis. Here the virus uses the uptake of material from the surrounding of the cell to get inside. This can either happen together with resources inside a vacuole. Or it happens via the binding of specific receptors, which are internalized into the cell upon binding of the specific ligand. Here the virus mimics the ligand of the receptor to get internalized into the cell. In both cases the virus needs to escape from the vacuole before it get broken down because the vacuole is turned into a lysosome.

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Thanks! I was talking about viruses which inject their genetic material; I just somehow forgot to mention that. =) –  evamvid Mar 10 at 17:27
    
Ah, ok. I answered that, too. :-) –  Chris Mar 10 at 17:32
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I think the external protein coat is degraded as the virus releases it's nucleic acid into the Host' cell. This is due to the fact that a protein is lifeless without the activity of a gene(one gene-one enzyme hypothesis).Since the nucleic acid is composed of genes, when it is injected into the host's cell, the capsid becomes useless and so it is degraded.

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Can you add some references? –  Cornelius Jun 7 at 15:44
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