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Lecture notes of 9 pages for the course Membrane and Cellular Biochemistry at QMUL
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Protein import and export through membranes in eukaryotesLearning objectives:
1. Understand the similarities and differences in protein targeting and translocation at the ER and bioenergetics
organelles (mitochondria and chloroplast)
2. Know how and where the membrane components of eukaryotic cells are synthesised and integrated into membrane
Protein import and export in eukaryotes
• In prokaryote, protein import and export is different
• Gene in the nucleus encodes MRNA, which is exported to the cytoplasm where protein synthesis is initiated
• Where could proteins go?
- A membrane protein could go to the
Plasma membrane
Golgi complex
ER
Nuclear envelop
- A water-soluble protein could
Stay in cytoplasm
Leave cell (secretion)
Nucleus
Lysosome
Other organelles (mitochondria, chloroplast, peroxisome)
• To go these destinations apart from staying in the cytoplasm, the protein must cross 1 or more membranes.
How is the protein targeted to the membrane and how does it get across?
• Getting a soluble protein across a membrane means moving charged residues through the hydrophobic interior of the
membrane
- Hydrophilic channel needed
• Most translocation system transfer the protein in an unfolded state
- i.e. long thin polypeptide chain, not globular mature protein
- This is easier because the channels do not have to be so wide.
1. Synthesis of secretory protein or a lysosomal enzyme
• The protein must cross the membrane of the RER, ending up in the RER lumen.
• secretory proteins are not initially targeted to the plasma membrane
• secretory proteins are targeted to the RER
View of ER from immunofluorescence
• Immunofluorescence
- Common laboratory technique, which is based on the use of specific antibodies, which have been chemically
conjugated to fluorescent dyes. These labelled antibodies bind directly or indirectly to cellular antigens
(primary or secondary IF)
, View of the RER from scanning EM view of RER from transmission EM
Model of RER
How is the protein directed this way?
• Protein synthesis starts with a ribosome and MRNA in the cytoplasm
• The polypeptide chain of the protein begins to be synthesised with the N-terminus appearing first
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