9 Interesting Facts About Protein Synthesis and Secretion for Nerds
1. Ribosomes
Ribosomes in cytosol and on rough endoplasmic reticulum are identical and in equilibrium.
Individual ribosome localizes itself on cytosol or endoplasmic reticulum, depending on which type of protein it is synthesizing.
2. Site of Protein Synthesis
Site of protein synthesis depends on where they are needed ultimately.
Cytosolic proteins are synthesized in cytosol by cytosolic ribosomes.
Secretory proteins and membrane proteins are synthesized in rough endoplasmic reticulum.
This is because once the protein is synthesized, it cannot enter or cross the lipid bilayer.
3. Secretory Pathway
Secretory and membrane proteins are synthesized in rough endoplasmic reticulum
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travels to the outer side of the endoplasmic reticulum
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Packed in transport vesicles
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Goes to cis Golgi i.e. the innermost saccule in the Golgi complex
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passes through each saccule of Golgi complex one by one and are further processed during this
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Reaches the trans-Golgi i.e. outermost saccule of the Golgi complex
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Proteins are sorted and packed in different vesicles according to their final destination
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Secretory proteins are packed in secretory vesicles → stored till needed → secreted when the appropriate stimulus comes.
Membrane proteins are packed in separate vesicles → merge with the target organelle.
4. No Mixing
During the entire process, no mixing occurs between cytosolic and luminal content.
e.g. secretory proteins are always confined within membrane-closed structures throughout its passage i.e. endoplasmic reticulum → transport vesicles → Golgi apparatus → secretory vesicles.
5. Resident Protein
Resident proteins on any organelle do not get swept along the flow.
i.e. resident protein of endoplasmic reticulum stays on endoplasmic reticulum and resident proteins of Golgi apparatus stay on Golgi apparatus.
6. Transport of Vesicles
Transport vesicles are transported along microtubules of cytoskeleton.
This movement is carried out by motor proteins like kinesin. Their one end holds the vesicles. And by the other end, they walk on the microtubule.
7. Topology of Membrane Protein
Topology is orientation of membrane protein across the membrane.
Proper orientation is necessary for protein to work properly.
E.g. for proper functioning of Na-K ATPase pump, its ATP binding domain should face inside the cell.
Membrane proteins are inserted in the membrane in an appropriate orientation right at the time of synthesis.
This is because once the protein is inserted in the membrane, it cannot flip.
8. Faulty Proteins
If misfolded or unassembled protein is produced
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It's tagged with ubiquitin
↓ Protein is removed from endoplasmic reticulum by a special process called retrotranslocation
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Proteosomes degrade these faulty proteins
9. Back Flow of Membrane
When a vesicle pinches off of any organelle, it takes a portion of membrane of that organelle → When it fuses with the target organelle, the membrane becomes part of that organelle.
e.g. transport vesicles take membrane from endoplasmic reticulum to Golgi apparatus. And secretory vesicles take membrane from Golgi apparatus to cell membrane.
If it continues forever, the size of donor organelles decreases, and the acceptor organelle increases.
To prevent this there is a system that brings the membrane portion back to its original source. So that size of each organelle is maintained constant.
Pinocytosis contributes to this backflow of membrane.