Section 6A: Fundamentals of cell compartments and protein sorting Flashcards
Proteins have an address built into them telling them where to go
like a “distribution centre”
Lysosome
breakdowns macromolecules
ER
builds up macromolecules
THES
condition that manifests in intestinal epithelial cells
- proteins that are supposed to be in one part of the cell end up in a different part
- protein DOES NOT make it to apical plasma membrane
- protein is involved in nutrient transport thus, if not in right location, protein CANNOT take up nutrients
Why do we care about protein sorting within cells?
because when they don’t localize properly, it can lead to disease (e.g. THES)
NHE
Na/H+ transporter has to be sorted by a cell to the apical plasma membrane
Membrane-bound organelles
contents are held together by the surrounding lipid bilayer: separates the inside from the outside
Membrane-less organelles
no surrounding membrane
How can membrane-less organelles stay together and disperse?
membrane-less organelles form because of Scaffold Molecules (proteins)
What are the characteristics of Scaffold Molecules?
-sticky: binds to other scaffold molecules
- floppy/loose: no secondary structure
- liquid phase: but separate when found
What are Client Molecules?
- these proteins can have secondary structures (α/β) that stick to the scaffold molecules
- allow to concentrate or couple certain types of proteins to work together
Some membrane-less organelles are also called __________
Biomolecular Condensates
Scaffold Proteins are made up of intrinsically disordered regions (no secondary structure) and interact with various non-covalent interactions:
- electrostatic
- Pi-pi (most favoured, ring structure with delocalized electron interacting)
- cation-pi
- hydrogen bonds
- van der waals interactions
What is the Nucleolus?
- an example of a membrane-less organelle
- where rRNA is processed and assembled into ribosomes
- made up of several “layers” of membrane-less organelles
Membrane-bound organelles have 3 distinct parts
- Lumen: the aqueous fluid enclosed by the lipid bilayer of an organelle
- The cytoplasmic face/leaflet of the bilayer: the outer membrane surface that faces the cytosol
- The lumenal face of the bilayer: the inner membrane surface that faces the lumen. constantly tidy up/sort proteins to right location or else materials get mixed up
How are proteins targeted and transported to specific organelles?
proteins have a built-in signal that tells them where to go
What type of molecules can diffuse across a lipid bilayer?
small non-polar, hydrophobic molecules can diffuse through a lipid bilayer
No protein on its own will go through a lipid bilayer:
- it needs a transport mechanism
- proteins that are responsible to transfer other proteins into an organelle
How do proteins enter membrane bound organelles?
- Ribosome translating mRNA into polypeptide
- Soluble proteins (aka anything other than membrane proteins) get sorted to specific organelles after they are made
OR - Transmembrane/Secreted proteins get sorted to ER while/as they’re being made
What causes organelle specialization?
- Unique integral membrane proteins
- Unique proteins in the organelle lumen
- Unique peripheral proteins on the organelle surface
- Unique lipids
Three important facts about
organelle function
- Each organelle has a specific set of functions
- Each specialized function requires a specific set of
proteins and lipids - Organelles are not isolated: they exchange content
If organelles are not isolated, how do you keep them separate?
signal sequences act as postal codes to target proteins to the correct membrane-bound organelle
- series of positively (+) charged amino acids to send a protein to the nucleus (e.g. Histones)
Where does translation of most proteins occur?
Cytosol
How to target proteins to a specific organelle?
Signal peptide sequences target proteins to specific
compartment
How to transport proteins to a specific organelle?
- Delivery across a “gate” (Gated Transport): between cytosol and nucleus, controlled by nuclear pore
- Delivery through a translocator/channel (Transmembrane Transport): between cytosol and ER, insertion of proteins (squeeze through) into membrane and lumen during translation by ribosome
- Delivery by transport membrane vesicles (Vesicular Transport): between ER, golgi, PM, endosomes, and lysosomes, transport membrane enclosed vesicles
