Lecture 10 - Endomembranes ER import Flashcards
(93 cards)
Where do all proteins begin synthesis?
ribosomes in the cytosol
Where do proteins need to end up?
- in the cytoplasm
- in an organelle
- in the plasma membrane (or ER/Golgi)
- outside the cell (secreted
What are the 2 protein sorting pathways?
- Secretory pathway (Co-translational import)
- Non-secretory pathways (post-translational import)
What are the 2 distinct locations where polypeptides are synthesized?
- 1/3 of proteins synthesized at the Rough ER
- secreted proteins
- transmembrane proteins
- soluble proteins residing in ER, golgi, lysosomes, endosomes, vesicles, vacuoles (endomembrane system) - remaining proteins are synthesized on ‘free’ cytosolic ribosomes
- proteins destined to remain in cytosol
- peripheral proteins of the cytosolic surface of membranes
- proteins transported to nucleus
- proteins incorporated into peroxisomes, chloroplasts, and mitochondria
Explain Proteins in Co-translational import
- proteins carrying an ER signal sequence that direct the ribosome-polypeptide complex to the RER
- translation is completed on RER
- proteins processed and sorted in the ER and Golgi
Explain Proteins in Post-translational import
- proteins lacking ER signal sequence
- therefore, complete synthesis on ribosomes that are free in the cytoplasm
- proteins released into cytoplasm and ones with an organelle-specific sorting signal are imported into the organelle
- cytoplasmic proteins do not have sorting signals and remain in the cytoplasm
What are the 4 important points to consider with respect to protein targeting?
- Sorting Signal
- tells proteins where to go as they are being synthesized
- information is coded in primary sequence - Receptor for the Signal Seq.
- Nature of the Translocation Channel
- Source of Energy
give rough overview of the endomembrane system and what each does
- Rough and Smooth ER and the Golgi are site for protein (and lipid) synthesis, processing, and sorting
- Endosomes carry and sort materials brought into the cell
- Lysosomes digest ingested material and unneeded cellular components
- Transition, Transport, and Secretory Vesicles move mol. btwn the compartment and plasma membrane
Explain structure of ER
- has tubular membranes and flattened sacs (cisternae)
- internal space is called lumen
- luminal space of the rough and smooth ER is contiguous (ex. they are not separate organelles and don’t need vesicle transport to move cargo btwn them)
What is Co-Translational Import (ER-docked ribosomes) used for?
- used by ribosomes synthesizing polypeptides destined for export from the cell and intracellular compartments (ex. lysosomes, endosomes, golgi)
- these ribosomes are attached to ER early in translation, and polypeptides are transferred across ER membrane as synthesis takes place
- an ER signal seq. directs polypeptide/ribosome to ER membrane
how are some polypeptides deposited into the ER lumen?
- co-translational translocation of soluble proteins deposits the polypeptide into the ER lumen by a ribosome that is attached to the ER membrane
Who got the 1999 Nobel Prize in Medicine and for what?
- Gunter Blobel
- found that proteins have intrinsic signals that govern their transport and localization in the cell
Explain the Signal Hypothesis
- address codes
- proposes that intrinsic mol. signals determine the localizations of some polypeptides
- if this signal is deleted, targeting of protein is lost
- if signal is transplanted (to another protein), targeting is passed on to recipient protein
How long is the ER signal seq. and what does it direct it to?
- 15-30 aa long
- direct the complex (mRNA/polypeptide/ribosome) to the RER surface
ER signal seq. are usually very different. What is the little thing that they have in common
- seq at the N-terminus usually has a core of 6-12 hydrophobic aa
- a hydrophilic, often positive region, proceeds the hydrophobic core
What is the signal sequence recognized by?
the Signal Recognition Particle (SRP)
What stops translation/protein synthesis?
- stops when the ER signal seq. has been formed
- the SRP binds to this and blocks further translation
What happens when SRP binds to the ribosome?
- directs ribosome to a structure in the ER membrane called the translocon
What is the SRP comprised of?
- 6 proteins and 300 nucleotide RNA
What are some examples of SRPs?
Note: all ‘P’ can be ‘SRP’ instead. (ex. P54 = SPR54)
- P54: binds ER signal seq.
- P68/P72: required for protein translocation
- P9/P14: Interact with ribosomes
Where does the complex bind the SRP receptor?
- binds the receptor, which is an integral membrane protein, on the cytoplasmic surface of the ER membrane, in close proximity to a translocon
What happens once the SRP is released?
- the ER signal seq is inserted into the translocon (channel protein in membrane)
What happens if SRP is not released?
- binding halts translation and blocks the nascent polypeptide from entering the translocon
What happens once the ER signal seq. enters the translocon?
- contact with the interior of the translocon displaces the plug, opening the channel to the ER lumen
- as the polypeptide elongates, it passed into the ER lumen
- a signal peptidase cleaves the seq, which is quickly degraded (final is 15-20 aa shorter)
- chaperones bind the nascent peptide to facilitate folding
- ribosome detaches from ER membrane, subunits dissociate and release mRNA and repeat!