Lecture 3 Flashcards
How much volume does the cytosol take up in the cell?
about 50%
How much volume does the mitochondria take up in the cell?
about 22%
How much volume does the ER take up in the cell?
about 12%
What percentage of the total cell membrane does the ER make up?
about 50-60%
What does the rough ER do?
- membrane-bound ribosomes
- synthesis of soluble proteins and transmembrane proteins for the endomembrane
What does the smooth ER do?
phospholipid synthesis, detoxification
What is the definition of an organelle?
a discrete structure or subcompartment of a eukaryotic cell that is specialized to carry out a particular function
What are some examples of membrane-enclosed organelles?
nucleus
ER
Golgi
What are some examples of organelles that are not membrane-bound?
nucleolus
centrosome
How are proteins sorted?
- cytosolic proteins: no sorting signal (default location is cytosol)
- Proteins with sorting signal: have a signal sequence (specific amino acid sequence)
How are specific proteins sorted to different organelles?
sorted by a signal sequence (a stretch of amino acid sequence in a protein) which directs the protein to the correct compartment. Each signal sequence specifies a specific destination in the cell, which is recognized by sorting receptors that take the protein to their destination.
What is post-translational sorting?
proteins are nuclear-encoded and fully synthesized in cytosol before sorting
How is transport through nuclear pores done?
Folded protein
- signal sequence (nuclear localization signal, NLS) is recognized by sorting receptor and binds
- sorting receptor helps bring the proteins through the nuclear pore into the nucleus
How are proteins sorted to peroxisomes?
by recognizing a specific targeting signal (SKL) at their C-terminus. This allows them to be imported directly into the organelle
Do proteins need to unfold for import into peroxisomes?
No, unlike mitochondrial and chloroplast import where proteins must be unfolded, proteins targeted for peroxisomes are imported in their folded state.
What is the state of proteins when imported into mitochondria and chloroplast?
unfolded
Where are most mitochondrial and chloroplast proteins encoded and where do their synthesis occur?
encoded in the nucleus and synthesized in the cytosol and targeted by a signal sequence for import
How do chaperone proteins like Hsp70 assist in protein sorting to mitochondria and chloroplast?
delay the folding of certain domains of a protein, keeping it unfolded until it can be imported
What is co-translational sorting?
proteins that are synthesized in the nucleus and have an ER signal sequence, which is associated with the ER during protein synthesis in the cytosol
mRNA arrive in the cytoplasm, translation starts on ribosomes in cytosol, proteins with ER signal sequence are inserted into the ER as translation continues
Why do proteins sort to the ER?
as an entry point to the endomembrane system (stay in ER or go to golgi, endosomes, etc)
How are proteins sorted into the ER?
sorted into the ER co-translationally; they are directed to the ER by specific hydrophobic signal sequences and translocated across or into its membrane.
What types of proteins are transferred from the cytosol to the ER?
water-soluble proteins: completely translocated across and released into its lumen
Transmembrane proteins: partly translocated across and embedded in its membrane
What steps occur during co-translational translocation of soluble proteins?
- translation start (N-terminal ER sequence emerges)
- ER sequence is recognized by signal recognition particles (SRP), protein synthesis stops for a bit
- SRP-ribosome complex binds to SRP receptor
- translocon opens
- proteins synthesis resumes with protein transfer into ER lumen
- signal peptidase cleaves ER signal sequence (which is hydrophobic, stays in lipid bilayer)
- protein is released into ER lumen
- translocon closes
What is the destination of soluble proteins after co-translational translocation?
lumen of an endomembrane organelle or secretion at the plasma membrane
What role does the signal recognition particle (SRP) play in co-translational translocation?
in targeting ribosomes that are synthesizing proteins with an ER signal sequence to the ER membrane. It binds to both the signal sequence emerging from a translating ribosome and to an SRP receptor on the ER membrane. This interaction pauses translation and facilitates transfer of the ribosome-nascent chain complex to an available protein-conducting channel in the ER.
How does translation resume during co-translational translocation?
Once bound to its receptor on the ER membrane, SRP releases from both its binding sites allowing for GTP hydrolysis and dissociation from its receptor. Translation then resumes with simultaneous threading or “translocating” of nascent polypeptides into or across membranes.
What steps occur during the co-translational translocation of transmembrane proteins?
- translation start (N-terminal ER sequence emerges)
- ER sequence is recognized by signal recognition particles (SRP), protein synthesis stops for a bit
- SRP-ribosome complex binds to SRP receptor
- translocon opens
- proteins synthesis resumes with protein transfer into ER lumen
- stop-transfer sequence enters translocon
- protein transfer stop and transmembrane domain released into lipid bilayer
- signal peptidase cleaved ER signal sequence and translocon closes
- protein synthesis is complete