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Most proteins are mature and functioning when they have been
synthesized. However, there are proteins, which still need to be
modified by either clipping off a part of the protein or by adding
additional elements. To show the importance of protein modification
we will look at the RAS
protein. This protein is often mutated in cancers and this
contributes to uncontrolled growth. To function properly RAS is
attached to the cell membrane. However, this is only possible when
a lipid (fat) soluble group is attached to its end (COOH-end),
which is called Prenylation. The attachment is carried out
by an enzyme called Farnesyl Transferase Protein. The
enzyme, the RAS protein and the prenyl group form a
complex and the prenyl group is chemically
linked to the C-terminal end. This is possible, because the
enzyme recognizes a certain sequence, which is unique for RAS.
Otherwise every protein would have this modification. There are
about 20-40 different proteins in the cell, which also have this
modification. Researchers have developed drugs to inhibit this
attachment to prevent mutant RAS proteins from functioning in
cancer cells. We will cover this topic in the chapter about drug
targets. Once RAS is prenylated it can attach to the membrane and
function in signal transduction. There are other protein
modifications, which are essential but we are not going into
further details. To summarize gene expression have a look at this
little
animation starting from transcription.
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