L8. Intracellular compartments & transport I Flashcards
main function of the endoplasmic reticulum
- synthesis of most lipids
- synthesis of proteins for distribution to many organelles and to the plasma membrane
main function of the Golgi apparatus
- modification, sorting, and packaging of proteins and lipids
- for either secretion or delivery to another organelle
main function of lysosomes
break down different biomolecules
main function of peroxisomes
break down of toxic molecules using H2O2
explain the evolution of the eukaryotic endomembrane system
an ancient prokaryotic archaea with an engulfed bacteria invaginates and creates the nuclear envelope
what are the three mechanisms of protein transport
- transport through nuclear pores
- transport across membranes
- transport by vesicles
mechanisms of protein transport - nuclear pores
- proteins moving from the cytosol into the nucleus
- protein needs to be folded
mechanisms of protein transport - across membranes
- moving from the cytosol into the ER, mitochondria, or chloroplast
- transported by protein translocators
- protein needs to be unfolded
mechanisms of protein transport - by vesicles
- moving onward from the ER, from one compartment to another
what are signal sequences
- it is a sorting signal on a protein
- it is both necessary and sufficient to direct a protein to a particular destination
transport through nuclear pores - explain the structure of the outer nuclear membrane
- it is continuous with the ER membrane
- the nuclear membrane is penetrated by nuclear pores
transport through nuclear pores - what are nuclear pores
- they form the gates through which molecules enter or leave the nucleus
- only allows small, water-soluble molecules to pass freely
- large molecules will need to have the appropriate sorting signal
transport through nuclear pores: nuclear pores - what is the appropriate sorting signal
- nuclear localization signal (NLS)
- NLS is recognized by nuclear import receptors
transport through nuclear pores - explain the import of proteins into the nucleus
- nuclear import receptors recognize nuclear localization signal (NLS)
- entry into nuclear pore causes the cargo to be released
- the receptor then returns to the cytosol
- this is all mediated by GTP hydrolysis
transport through nuclear pores - explain how nuclear transport is mediated by GTP hydrolysis
- as the nuclear import receptor takes in a protein into the nucleus, it encounters Ran-GTP
- Ran-GTP binds to the import receptor and causes it to release the nuclear protein
- the receptor (still carrying Ran-GTP) is taken out of the nucleus
- outside, an accessory protein hydrolyzes the GTP turning it to Ran-GDP
- Ran-GDP then falls off the import receptor
transport through nuclear pores: nuclear transport is mediated by GTP hydrolysis - what is Ran-GAP
- it is in the cytosol and causes Ran-GDP to predominate in the cytosol
- GTPase-Activating Protein
transport through nuclear pores: nuclear transport is mediated by GTP hydrolysis - what is Ran-GEF
- it is in the nucleus and causes Ran-GTP to predominate in the nucleus
- Guanine Exchange Factor
transport through nuclear pores: nuclear transport is mediated by GTP hydrolysis - what is Ran
monomeric GTPase
transport across membranes: mitochondria - explain how proteins are unfolded during import to the mitochondria
- both outer and inner membranes must be crossed
- receptor recognizes the mitochondrial signal sequence
- the receptor then interacts with a protein translocator in the outer membrane
- the protein/receptor/translocator complex diffuses in the outer membrane and encounters the inner membrane translocator
- the two translocators then transport the protein across both membranes while unfolding the protein
- once in the mitochondrial matrix, the signal sequence is cleaved off
transport across membranes: ER- how is the ER network extensive
- most proteins begin to be threaded across the ER membrane before the polypeptide chain has been fully synthesized
- this means that the ribosome synthesizing the protein has to be attached to the ER membrane (rough ER)
transport across membranes - how are common pools of ribosomes used to synthesize proteins
- ribosomes remain in cytosol when they are translating proteins with no ER signal
- ribosomes will be directed to the ER when they encounter a ER signal sequence
- these ribosomes will bind to each mRNA to form a polyribosome
- at the end of protein synthesis, the subunits are released back into the cytosol to rejoin the common pool
transport across membranes: ER - what must happen for growing polypeptide chain to go into the ER
- a signal recognition particle (SRP) binds to the ER signal sequence and ribosome
- this causes protein synthesis to be slowed down
- the ribosome/protein/SRP complex binds to the SRP receptor on the ER
- this causes the SRP to be released and the ribosome will be passed to a translocator
- protein synthesis will then resume in the ER
transport across membranes: ER - what happens after protein synthesis is resumed
- a translocator binds to the signal sequence and threads the protein through as a loop
- the signal peptide will be cleaved by a signal peptidase and will be degraded
transport across membranes: ER - what happens after protein synthesis is finished and the polypeptide becomes a soluble protein
- the soluble protein is released into the ER lumen
- the translocation complex pore closes
transport across membranes: ER - explain how a single-pass transmembrane protein is retained in the bilayer
- the N-terminal signal initiates translocation
- a second hydrophobic sequence act as a stop-transfer sequence
- as the stop-transfer sequence enters the translocation channel, the channel discharges the growing protein sideways
- the N-terminal signal peptide is cleaved and protein synthesis continues in the cytolytic side
transport across membranes: ER - explain how a double-pass transmembrane protein is retained in the bilayer
- internal signal sequence acts as a start-transfer sequence and as an anchor
- second hydrophobic sequence acts as a stop-transfer sequence
- when stop-transfer sequence enters the translocation channel, the channel will discharge both the signal sequence and the stop-transfer sequence into the bilayer
- this causes the protein to be grown sideways
- neither sequences are cleaved and protein synthesis will continue on the cytolytic side
transport across membranes: ER - single-pass N-C terminus orientation
- in ER lumen: C in cytosol, N in organelle
- in cytosol being transported to membrane: N will move towards the outside of the cell with C staying in the cytosol
transport across membranes: ER - double-pass N-C terminus orientation
C and N always faces the cytosol
