Membrane organelles Flashcards
What did the invagination of the plasma membrane cause?
evolution of nuclear membranes, ER, Golgi appratus, endosomes and lysosomes
How did mitochondria and chloroplasts evolve?
Likely evolved from engulfed bacteria by primitive eukaryotic cells where they lived in symbiosis.
Where are proteins made in the cell?
From cytosol and are either given directly from cytosol (mitochondria, chloroplast, interior of nucleus) or indirectly via ER (golgi body, lysosomes, endosomes, inner nuclear membrane)
How does a protein in the cytosol know where to go?
Sorting signal in amino acid sequence directs it to a certain destination
How does a water soluble protein made in the cytosol enter an organelle?
- Nuclear pores: nuclear proteins
- Protein translocators: proteins entering ER, mitochondria or chloroplasts
- Fusion of transport vesicle with membrane of destination: proteins exiting ER
What is the difference between transport through nuclear pores and protein translocators?
Nuclear pore protein’s shape remains intact
Translocator transport requires unfolding to cross membrane
What is the nuclear localization signal?
Signal sequence recognized by nuclear import receptors that directs entry of protein through nuclear pore into the nucleus
Lys-Lys-Lys-Arg-Lys
How are proteins delivered to the nucleus?
- Nuclear localization signal is recognized by nuclear import receptors
- Receptors bind and guide proteins to the pore by interacting with fibrils that extend from the rim
- Receptors jostle through gel like mesh of pore
- Entry into the nucleus triggers cargo release
What is Ran?
GTPase that aids in cargo transport into the nucleus through nuclear pores. Ran-GTP is found most in the nucleus, and Ran-GDP is found most in the cytosol
How does Ran work?
- After cargo is sent to nucleus, Ran GTP displaces the cargo on the import receptor due to higher affinity
- Import receptor returns to the cytosol with Ran GTP attached
- Ran GTP is hydrolyzed in cytosol to Ran GDP
- Ran GDP has low affinity for import receptor causing dissociation and displacement with protein with higher affinity to be sent to nucleus
What is Ran GAP and Ran GEF?
Ran GAP: found exclusively in the cytosol, hydrolyzes Ran GTP to Ran GDP and maintain cytosolic [GDP]>[GTP]
Ran GEF: found exclusively in the nucleus: exchanges GDP for GTP and maintains nuclear [GTP]>[GDP]
How do proteins enter mitochondria and chloroplasts?
- Precursor protein has signal sequence recognized by import receptor protein
- Protein translocator in outer membrane (TOM) transports signal sequence to intermembrane space
- receptor, precursor protein and TOOM diffuse until signal is recognized by translocator in inner membrane (TIM)
- Together TIM and TOM unfold the protein and transport the protein across both membranes simultaneously
- Signal sequence is cleaved by peptidase in mitochondrial matrix and refolded with help of chaperone proteins
How do unbound ER proteins enter the ER?
- Signal recognition particle in cytosol binds to a ribosome that is creating the hydrophobic signal sequence, inhibiting synthesis until binding with the ER receptor
- SRP receptor on ER membrane recognizes and binds to the signal recognition particle
- Once bound to receptor, SRP releases ribosome allowing it to transfer to a protein translocator and continues synthesis
- Polypeptide threads across ER membrane into lumen during synthesis as signal sequence is stuck at protein translocator
- After synthesis, peptidase cleaves signal sequence which is degraded into the lipid bilayer.
How does translocation of single pass proteins differ from multipass proteins?
Singlepass: hydrophobic amino acid further along the chain halts transfer process and continues synthesis outside the lumen and start is cleaved.
Multipass: Start transfer sequence is not cleaved and synthesis continues into the lumen until a hydrophobic sequence is reached.
What features does an open ER protein translocator have?
Allows protein to move across the membrane through the central hydrophilic channel or move into lipid bilayer via lateral gate
where are water soluble proteins released from the ER?
Released at cell surface by secretion or retained in lumen of an organelle of the endomembrane system
Where are traansmembrane proteins released from the ER?
Reside in the membrane of one organelle of the endomembrane system or in the plasma membrane
How do endomembrane system organelles get lipids?
Vesicular transport
How do mitochondria and other organelles get lipids?
Lipid carrying proteins transfer phospholipids between membranes via close-contact junctions between organelles
What is the secretory pathway of vesicular transport?
Moves materials from ER to plasma membrane and secreted via exocytosis
What is the retrieval pathway?
Vesicle pathways that carry back ER proteins that have escaped to the golgi apparatus
What is clathrin?
Protein coat on cytosolic side of the vesicle that aids in the formation of vesicles
How does the budding mechanism of clathrin coated vesicles work?
- Cargo binds to cargo receptors with the help of adaptins
- Adaptins bind to clathrin protein coat
- Clathrin forms bud
- Dynamin pinches neck creating a clathrin coated vesicle
- In the cytosol clathrin is released
How does a transport vesicle recognize its target organelle?
- Rab proteins on surface of each type of vesicle are recognized by tethering proteins on the surface of target membrane
- v-SNARE interacts with t-SNARE on target membrane
- v-SNARE and t-SNARE draw two lipid bilayers close to eachother
- both SNARE’s wind together to squeeze water molecules trapped between two membranes
