Lecture 8 - Intracellular Compartments + Protein Sorting Flashcards
Major compartments/organelles of endomembrane system of eukaryotic cells
- Lysosome
- Golgi apparatus
- Mitochondrion
- Endoplasmic Reticulum
- Nucleus
- Plasma membrane
- Peroxisome
Lysosome function
Protein destruction
Golgi Apparatus function
Protein modification and export
trans golgi faces plasma membrane
Mitochondrion function
Energy production
Endoplasmic Reticulum function
Smooth: lipid production, detoxification
Rough: protein production, particularly for export out of cell
Nucleus function
DNA storage
Peroxisome function
Lipid destruction
Endosymbiosis theory and what organelles are present based on this.
Anaerobic eukaryote took in aerobic prokaryote, evolution caused aerobic bacterium to multiply w/in => anaerobic eukaryote and aerobic prokaryote form symbiotic relationship => membrane of mitochondria/chloroplast derived from eukaryotic cell host
3 major modes of protein transport
- Gated transport
- Transmembrane transport
- Vesicular transport
What is gated transport?
The movement of proteins b/t cytosol and nucleus (nuclear pore complexes: display specificity), also import of transcription factors, nuclear factors, DNA pol, etc.
What is transmembrane transport?
- Proteins transported from cytosol to mitochondria, ER, plastids, or peroxisomes
- Facilitated by TM protein translocators
- Cargo unfolds to pass through translocator
What is vesicular transport?
- Proteins transported from ER to Golgi, Golgi to late endosome/lysosome/early endosome/cell exterior, cell exterior, secretory vesicles/cell exterior
- Membrane-enclosed transport intermediates ferry proteins from one compartment to another via membrane fusion.
- Proteins transported this way never cross membranes
Properties of lipid vesicles and vesicular transport
Vesicles bud and fuse during vesicular transport b/c membranes share structural and physical properties
Signal sequences: function. Don’t memorize actual AA sequences of these, though! Where located?
Serve as indicator of where proteins should be targeted
On N terminus
Nuclear import from cytosol: features of the nucleus and nuclear envelope
Features of nucleus/nuclear envelope:
- double membrane
- contiguous w/ ER
- has nuclear pore complexes -> gateways through which things get into/out of nucleus (gated diffusion barrier via size, active transport for larger molecules)
- Nuclear lamina network of intermediate filament provide structural integrity to nucleus
What are nuclear localization signals?
Signals that act as ID for molecules to enter nucleus
What do nuclear import receptors bind?
NPC proteins and nuclear localization signals on cargo proteins
Nuclear import from cytosol: role of Ran proteins, GTP hydrolysis, regulation
GTP-binding proteins that facilitate nuclear transport
GTP hydrolysis by Ran proteins provide directionality to nuclear transport
Regulated by type of bound guanine nucleotide => cycle b/t GDP/GTP bound forms via exchange reactions
Nuclear import from cytosol: control of nuclear envelope formation/breakdown (slide 19).
- Dynamic assembly/disassembly of nuclear lamina is important for things like cell division
Transport of proteins into mitochondrion: features, requirements, steps.
Two mitochondrial protein translocators:
- TOM complex: receives protein
- TIM23 complex: spans both membranes, brings protein into matrix
Requires:
- ATP hydrolysis
- Membrane potential
Steps:
- Protein has a signal sequence that binds to import receptors
- TOM Complex has receptor, triggers insertion into membrane
- TIM23 Complex translocates protein into matrix
- Signal is cleaved by signal peptidase
Transport of proteins from the cytosol into ER: features, components, steps.
2 ways: Co-translational (destined for ER) or Post-translational translocation (destined elsewhere)
Co-translational:
- Ribosome translates mRNA, produced protein has signal
- SRP binds to signal peptide => pause translation
- SRP-bound ribosome attaches to SRP receptor in ER membrane
- Translation continues and translocation begins
- SRP and SRP receptor displaced/recycled
Differences b/w SER & RER, how they can be isolated.
Differences: SER microsomes have low density and stop sedimenting/float at low sucrose concentration; RER microsomes have high density and stop sedimenting/float at high sucrose concentration
Separation: High speed centrifugation w/ density gradient
The signal hypothesis of protein translocation, the SRP
The signal hypothesis of protein translocation: translocation is signal sequence specific, and the signal determines target compartment
Signal-recognition particle (SRP): directs ER signal sequences to a specific receptor in the RER membrane
Translocation of soluble proteins from cyt to ER
- Protein w/ signal attaches to inactive protein translocator => active translocator
- Signal peptidase cleaves signal
- Mature soluble protein in ER lumen
Translocation of single TM proteins w/ terminal ER sequence from cyt to ER
- Signal acts as start transfer sequence, stop-transfer sequence is internal
- Translocator translocates protein until it hits stop-transfer sequence => signal peptidase cleaves signal
- ER communicates w/ other organelles => bud off protein in membrane as vesicle to transport for destined location
Translocation of single TM proteins w/ internal ER sequence from cyt to ER
- The sequence acts as both the start and stop sequence, w/ the protein orientation dependent on the location of flanking positively charged amino acids w/ respect to internal ER sequence
Translocation of 2 TM domain proteins from cyt to ER
- Signal sequence is internal and acts as start-transfer, stop-transfer also internal
Translocation of multiple TM domain proteins from cyt to ER
- Multiple internal start and stop sequences in protein
Glycosylation of proteins: what is it? Functional importance?
- Protein modification during folding => attachment of sugars to specific AA residues
Glycoslyation can be either N-linked (asparagine) or O-linked (hydroxylate AA like serine, threonine)
Function: important for ensuring eventual folding of protein in ER lumen, if not properly folded => ubiquitination (degradation of protein)
Degradation of misfolded proteins.
- Misfolded protein is brought to protein translocator
- In cytosol, ubiquitin attaches to lysine residues and protein is brought to proteasome where proteins die
GPI-linked proteins
Some proteins are linked to glycosylphosphatidl-insoitol (GPI), anchoring them to membrane
