Richard.DeWald@f70.n382.z1.fidonet.org (Richard DeWald) (07/16/90)
Since no one else was answering the inquiry from the person who was curious
about the "viral protein shell," I thought I would try my hand at trying to
answer the question. Since then, I have been innudated with large volumes of
email from people who were correcting the specifics of what I said.
Caveat emptor: I learned what I know about virii from a course designed for
general information about virii. The thrust of my post was correct, I just
missed a few details. I will correct those here and now.
Mistake numero uno:
HIV is a retrovirus. I knew this, and I knew the process wasn't as simple as I
wanted it to be. What defines a retrovirus is the fact that the viral RNA is
"reverse transcribed" in the cell into DNA. This viral DNA is then inserted
into the cell's DNA. This new altered DNA then carries out the altered
instructions that provide for the production of more HIV. The new altered DNA
must be transcribed into altered RNA before production of new HIV commences.
This DNA to RNA process is the way things normally work in the cell.
Other virii do in fact insert their RNA into the target cell's RNA, it was my
mistake that I had not made this distinction. I was trying to keep it simple.
What is the difference between DNA and RNA? Structurally they differ very
little, but a little difference is a major difference in molecular biology.
Generally, RNA is the "working copy" of cellular instructions, DNA is the
"archival copy." (I continue to use computer metaphors because everyone who
reads this has at least some knowledge of computers. Also, in most cases, the
metaphor works well.)
The distinction is also important for the pathologic course of the virus. As
long as the altered DNA is not transcribed into working RNA (as long as a new
working copy is not made from the corrupt archival copy) there is no
manifestation of disease.
Mistake numero dos (please, no one corect my spanish):
The exterior coat of the HIV virus is lipoprotein, not protein. Lipoprotein
is a protein/fat mixture. Lipoprotein is also what makes up the external
membranes of normal everyday cells all over the body. The lipid (fat) portion
of the viral exterior coat is dervied from the cell, and part of the protein is
derived from the cell as well. There is a glycoprotein (sugar/fat) molecule on
the surface of the exterior coat of the virus that is responsible for the first
attachment to the target cell's membrane.
The virus does have a protein shell, but that is inside of the exterior
membrane-like coat described above.
Mistake numero tres:
On the target cell, there is a receptor known as CD4 that is what the viral
glycoprotein is attracted to. The viral glycoprotein is the "key" to the CD4
membrane protein "lock." CD4 is found on the membranes of cells other than the
T-lymphocyte, so HIV does infect cells other than the T-lymphocyte. T-cells
get all of the attention right now becasue their absence is what cause all of
the havoc.
So, to summarize:
The HIV RNA is encapusulated by a protein shell. This shell is inside of a
lipoprotein membrane coat. The fact that the lipoprotein coat is mostly
derived from the cell of the infected individual is what makes it difficult for
that person's immune system to recognize that there is a foreign invader in
it's midst. It is sort of like an enemy spy wearing his target's uniform.
This spy has a badge on that allows him access to the inner workings of certain
cells, where he cleverly has the cell insert his altered instructions into the
archival copy of instructions kept by the cell. These instructions provide for
the synthesis of new spies who are then issued friendly uniforms by the cell as
they leave. This doesn't happen immediately because working copies of those
instructions are still in place in the cell. The new spies are only made once
a new working copy of the instructions is made from the now corrupt archival
copy.
Any gross attempts to disrobe the spies would also disrobe the friendly
soldiers who are wearing almost identical uniforms. There are attemps underway
now to chemically alter the badge that the virus wears rendering it useless for
gaining access to new cells. THAT is why the external coat of the virus is
currently a hot topic.
Further corrections are welcome. My humble thanks to Dr. Ernest Retzel of the
University of Minnesota for his patient correction of my over-simplification.
The quiz will be on Monday.
Richard DeWald, BSN Student
Univ. of TX - Austin.
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