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Summary Biogenesis of membrane proteins in prokaryotes
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Voortgezette Celbiologie Evelien FloorBiogenesis of membrane
proteins in prokaryotes
Membrane proteins
Proteins in bacteria are made in the cytosol with ribosomes. After that, they can stay in the cytosol or
have a function in the outer or inner membrane. Some proteins are translocated to the outside of
the bacterium or to the periplasm.
30% of sequenced genes encode membrane proteins, you can predict this by looking for hydrophobic
regions. Membrane proteins are involved in vital cellular processes such as signal transduction,
communication and transport. 70% of drug targets are membrane proteins. Many diseases are due
to comprised folding or processing of membrane proteins. There are very few high-resolution
structures available. This is because it is very difficult to isolate a protein that is fixed in the
membrane. When a protein is isolated, the hydrophobic regions get exposed to a watery surrounding
and it loses its original structure.
E. coli is a good model organism because it is a simple organism and accessible to genetics (make
mutants). It is a widely used host for membrane protein overexpression. In E. coli the proteins can
only stay in the cytoplasm or in the inner membrane.
Both in eukaryotes and in prokaryotes the proteins consist of a signal peptide of 20-30 amino acids
long, with a central hydrophobic region. If you fuse a prokaryotic signal peptide to a eukaryotic
protein the protein goes to the ER, this also works vice versa. This indicates that signal peptides have
the same function in different organisms.
Proteins synthesized by the ribosome can move co-
translationally to the translocon. A chaperone can keep the
protein unfolded or it can help the protein fold. It is also
possible that a protein gets synthesized in the cytosol and a
chaperone will keep it unfolded. It can move to the translocon
afterwards. On the other side of the membrane the
chaperone will help the protein fold. The energy for this
process is provided by a motor protein. The translocon is also
used to get proteins into the membrane.
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