BIOL 130 Lecture 7: Biol 130 Unit 7 - Intracellular Compartment Transport

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7 Aug 2016
Unit 7 - Intracellular Compartments & Transport
For a cell to operate effectively, different intracellular processes that occur simultaneously must be
- One strategy: aggregate different enzymes required to catalyze a particular sequence of
reactions into large, multicomponent complexes
- Second strategy highly developed in eukaryotes: confine different metabolic processes and
proteins required to perform them in different membrane-enclosed compartments
Membrane-enclosed organelles
Protein sorting - each compartment contains a unique set of proteins that have to be transferred
selectively from the cytosol where they are made, to the compartment they are used
- Depends on signals built into AA sequence
- Synthesis of virtually all proteins on free ribosomes
- Protein transport always requires energy
How certain membrane-enclosed compartments in a eukaryotic cell communicate with each other by
forming vesicles
-Vesicular transport - pinch off from one compartment, move through the cytosol
Main route from releasing proteins (exocytosis) and importing them (endocytosis)
Membrane-enclosed organelles
Eukaryotic cells contain a basic set of membrane-enclosed organelles
Membrane-enclosed organelles - held by microtubules, moved by cytoskeletal filaments
- Cytosol enclosed by plasma membrane
- Nucleus surrounded by nuclear envelope, double membrane
Communicates with cytosol via nuclear pores
- Endoplasmic reticulum consistent with outer nuclear membrane
Major site of synthesis of new membranes in the cell
Rough.smooth ER
- Golgi apparatus receives proteins and lipids from ER, modifies and dispatches to other
- Lysosomes digest worn out material
Transport from endosomes
- Mitochondria/chloroplast surrounded by double membrane, site of oxidative phosphorylation
or photosynthesis to produce ATM
Membrane-enclosed organelles evolved in different ways
Membranes of ER, golgi, endosomes, lysosomes: invagination of the plasma membrane
-Endomembrane system
- Interiors of these organelles are treated by the cell as “extracellular”
Mitochondria and chloroplasts - have own genomes, make some of their own proteins
- Isolated from vesicular traffic
- Endosymbiont theory
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Protein sorting
Before a cell divides, must duplicate membrane-enclosed organelles
- Requires supply of new lipids for new membrane, and proteins
Proteins for membrane and interior
Always being synthesized
For some organelles, proteins delivered directly from cytosol
- Others, delivered indirectly from ER, which is a major site of lipid and protein synthesis
Proteins enter ER from cytosol
Some retained, most transported by vesicles to the golgi, onwards to plasma
membrane or other organelles
How proteins directly enter membrane-enclosed organelles from the cytosol from their address label on
AA sequence
Proteins transported into organelles by 3 mechanisms
Synthesis of all proteins begins on ribosomes (except mitochondria, chloroplast)
- Fate depends on AA sequence sorting signal
When organelle imports water-soluble protein into interior, faces the problem of transporting protein
across membrane which is impermeable to hydrophilic macromolecules
- Cytosol into nucleus: nuclear pores penetrate inner and outer nuclear membranes
Very high traffic but flow is selective
Selective gates, active transport or diffusion of small molecules
- Cytosol into ER, mitochondria, or chloroplasts: protein translocators in membrane, but protein
must unfold
Signal sequence at N terminal has “address”
Protein translocator in inner membrane
- Proteins onwards from ER between compartments of endomembrane system: transport
Pinch off and fuse
Signal sequences direct proteins to the correct compartment
Signal sequence - often removed from finished protein once it is sorted
- Same destination can still vary even though they have the same function
Hydrophilicity, charges of AAs
Proteins enter the nucleus through nuclear pores
Nuclear envelope - encloses nuclear DNA, 2 concentric membranes
- Inner nuclear membrane - proteins that are binding sites for chromosome, anchor nuclear
lamina (protein filament network)
- Outer nuclear membrane - continuous with ER
Nuclear pores - disordered segments, prevent passage of large molecules but allow small, water-soluble
molecules to pass nonselectively
- Large molecules (RNA, proteins) must display appropriate sorting signal (nuclear localization
signal) with certain AAs
Recognized by nuclear import receptors
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Help direct newly synthesized protein by interacting with tentacle-like fibrils
extending into cytosol from the pores
Receptors interrupt interactions within the nuclear pore, form passageway, and
shuttles back and forth
Process requires energy - hydrolysis of GTP with Ran enzyme
Transport proteins in folded conformation - no unfolding!! - and complete ribosomes,
contrasting protein translocation
-Nuclear export signal tags protein for export
Proteins unfold to enter mitochondria and chloroplasts
Both organelles have inner and outer membranes and synthesize ATP
- Third membrane in chloroplast: thykaloid membranes
- Most proteins from genes in nucleus, imported from cytosol
Signal sequence near N terminus
Unfolded protein during transport, signal sequence removed
Chaperone proteins in organelles help pull protein, fold it inside
Growth and maintenance of mitochondria and chloroplast require not only protein import, but also new
- Imported from ER from lipid-carrying proteins, where two organelles are in close proximity
Proteins enter peroxisomes from the cytosol and ER
Peroxisomes - contain enzymes that break down molecules, synthesize certain phospholipids
Acquire bulk of their proteins via selective transport from the cytosol
- Protein translocator that its in transport, but protein does not need to unfold
- Also from ER from vesicular transport
Proteins enter the ER while being synthesized
ER is most extensive membrane system, is an entry point for proteins destined for other organelles as
well as the ER itself
- Proteins destined for golgi, endosomes, lysosomes, cell surface first enter ER
- In lumen, do not enter cytosol afterwards
Ferried by transport vesicles in endomembrane system or to plasma membrane
2 types of proteins transferred from cytosol to ER
- Water-soluble - completely translocated, released into lumen
Secretion at cell surface or lumen of another organelle of endomembrane system
- Prospective transmembrane proteins - partially translocated, embedded in ER membrane
Destined for membrane of endomembrane system or plasma membrane
Hydrophobic AAs
Unlike elsewhere, proteins that enter ER begin to be threaded across the membrane before the
polypeptide chain has been completely synthesized
-Rough ER - ribosomes atop membrane
Two separate ribosome species, same function, different protein production. Advantage that no
additional energy is required for transport as elongation of each polypeptide provides thrust to push
chain through ER membrane
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