5. Endoplasmatic reticulum and protein synthesis Flashcards
The endoplasmatic reticulum
- A network of membranous tubules within the cytoplasm, which is continuous with the nuclear membrane.
- First observed in 1945 (Porter, Claude & Fullam
Percentage of total cell volume: 12
Approximate number per cell: 1
Endoplasmatic reticulum
• Involved in protein and lipid synthesis • Present in most eukaryotic cells • Two types of ER – Rough ER – Smooth ER
- RER is studded with ribosomes and is involved in protein synthesis
- SER does not have ribosomes and mostly deals with lipid synthesis
Structure of the ER
- Membranous network of cisternae
- Phospholipid membrane encloses the cisternal space (lumen), which is continuous with the perinuclear space, but separate from the cytosol
- Relative quantities of RER and SER can change from one cell type to another, depending on changing metabolic activities
Protein synthesis
• Involves the translation of message (mRNA) into protein
– Performed by a ribosome
• Ribosome binds to the mRNA that has exited the nucleus
• tRNA, which have amino acids attached, work with the ribosome to add amino acids one at a time to build the new (nascent) protein
– There are 20 tRNA, one for each amino acid
The ribosome
• Molecular machine found in all living cells
• Primary function is protein synthesis
– Ribosomes link amino acids together to form proteins
• Composed of 2 major subunits (40S & 60S in eukaryotes)
– Small subunit reads the mRNA
– Large subunit adds amino acids to the growing polypeptide chain (catalysed by the catalytic peptidyl transferase activity of the subunit)
• Each subunit composed of one or more rRNA and a variety of proteins
– Collectively termed the translational apparatus
Translation specifics
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The tRNA + aminoacyl tRNA synthetase
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Polyribosomes
• Clusters of ribosomes that bind the same mRNA molecule • Can be found in three forms – Free – Membrane-bound – Cytoskeletal-bound
The ER + protein trafficking
• Secretory pathway – involves the ER
– Soluble and membrane proteins
- Proteins secreted from the cell
- Proteins resident in the lumen of the ER and Golgi complex
- Integral proteins in the membranes of the ER, Golgi & plasma membrane
• Non-secretory pathway
– Targeting to an intracellular organelle (mitochondria, nucleus, etc)
The secretory pathway (involves ER)
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Overview of secretory pathway
• Ribosomes synthesising proteins with an ER signal sequence are directed to the ER
– Facilitated by the signal recognition particle (SRP)
• After translation and translocation are complete in the ER, proteins are packaged into transport vesicles destined for the Golgi
– Further sorted for eventual delivery to the plasma membrane or lysosomes
Signal sequences
Import into ER:
- > +H3N - indication of N-terminus of protein
- > extended block of hydrophobic amino acid (e.g. Leu)
- > negatively charged (e.g. Glu)
Retention in lumen of ER:
- > positively charged (e.g. Lys)
- > negatively charged (e.g. Asp)
- > COO- - indication of C-terminus
Import into mitochondria:
- > +H3N - indication of N-terminus of protein
- > positively charged (e.g. Arg)
Import into nucleus:
-> positively charged (e.g. Lys)
Import into peroxisomes:
-> positively charged (e.g. Lys)
=> The ER retention signal commonly referred to by single-letter amino acid abbreviation, KDEL.
ER - targeted protein synthesis
- > mRNA encoding cytosolic protein remains free in cytosol
- > elongating polypeptide chain
- > polyribosome free in cytosol
¦
common pool of ribosomal subunits in cytosol
¦
ER membrane with the ER lumen
-> ER signal sequence
-> mRNA encoding a protein targeted to ER remains membrane-bound
=> polyribosome bound to ER membrane by multiple nascent polypeptide chains
Signal recognition particle function
mRNA signal sequence on nascent polypeptide + tRNA -> binding of SRP to signal peptide causes a pause in translation -> SRP bound ribosome attaches to SRP receptor in ER membrane
- SRP recognises signal sequence on nascent polypeptide
- SRP binding to signal sequence stalls translation whist complex moves to ER membrane and binds to SRP receptor
- SRP/Ribosome complex associates with SRP receptor on RE
SRP function
=> translation continues and translocation begins
=> SRP and SRP receptor displaced and recycled
- SRP docks with SRP receptor
- SRP transfers nascent peptide and ribosome to the translocon (Protein Translocator)
- SRP and SRP receptor dissociate and are recycled
