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Hi friends, in this video let us understand how membrane and secretary proteins are synthesized
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in the endoplasmic reticulum. Let's begin with a question for concept clarity. We know that RER is
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the site of membrane and secretary protein synthesis and it has ribosomes attached to the
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outer surface as you see here but the question is are ribosomes permanently attached to the rough
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interplasmic reticulum. Actually the ribosomes of the RER are not permanently attached to the
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membrane. They constantly attach and detach to the membrane as needed for protein synthesis
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Translation of secretory or integral membrane proteins initiates in the cytosol then ribosomes
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containing these mRNAs are recruited to the ER membrane and fuses with the membrane and finally
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releases the polypeptate to the ER lumen. Let us divide the entire process into
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simple steps for better understanding. Step 1, as you see this is mRNA that
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comes out of the nuclear pore into the cytosol or the cytoplasm. Then the
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ribosomes step 1 is ribosomes assembles on mRNA Larger subunit and smaller subunit of ribosome spines to the mRNA Step 2 is signal sequence synthesis in the amino acid terminus of nascent polypeptide So this ribosome starts
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synthesizing a signal sequence which is a hydrophobic amino acid sequence of 18
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to 30 residues that is recognized by signal recognition particle which is a
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protein. So step two you can see this ribosome starts in the sensing signal
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peptide. In step three you can see there are many free signal recognition
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particles that is present in the cytoplasm. It recognizes and binds to the
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nascent polypeptide ribosome complex. So this is a complex and this signal
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ribosome polypeptide complex and inhibits Werther translation. So protein synthesis is stopped
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Then this SRP or signal recognition particle is an RNA protein complex that is abundant in cytosol
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a universally conserved ribonuclear protein that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and in prokaryotes it targets to the plasma membrane Then step 4 is SRP then targets this entire complex As you see it is made up of ribosome a nascent
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chain complex, a polyptide with the signal peptide and that is bounded by SRP
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to the protein conducting channel that is located on the ER which is called as
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translecon. ER membrane as you see this is the translecon. This occurs via the
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interaction and docking or binding of SRP with its receptor. There is an SRP
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receptor that is located on ER membrane. SRP binds to this receptor. Step 5 upon
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docking or upon binding the nascent polypeptide chain is inserted into the
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translocone channel as you see here where it enters into the ER. Then protein
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synthesis resumes as SRP is released from the ribosome. Step 6 once inside the ER
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the signal sequence is removed from the core protein by an enzyme called signal
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peptides thus ensuring that signal sequence are not included or not there
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as part of the mature protein Step 7 the SRP receptor complex dissociates via GTP hydrolysis and the cycle of SRP mediated protein translocation to the ER membrane continues So once inside the
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lumen following protein synthesis and translocation inside the ER lumen a protein that is destined for secretion undergoes proper folding and
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modifications with the help of chaperones and folding enzymes. Modifications include disulfide bond formation and initial glycosylation and finally the
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folded processed protein is packaged in vesicles to Golgi apparatus and further
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modified there, processed there within the Golgi and secreted out of the cell by
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exocytosis. For more details you can refer our video on endomembrane system
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where we discuss both the pathway, the secretory pathway and also the pathway
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that is where lysosome is involved. So these are the steps involved in the
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synthesis of secretory and membrane proteins in ER or endoplasmic reticulum. hope you are clear take a step last thank you so much you are with biology
