Secretory Pathways 1,2, Nuclear Import Export, Protein Degradation Flashcards
Describe transmembrane and vesicular protein transport.
Transmembrane: Proteins move through translocators, across a membrane (e.g. cytosol → ECF, cytosol → mitochondria)Vesicular: Membrane bound transport intermediates move proteins and lipids from one compartment to another
List the major functions of the ER.
Synthesis of lipids (phospholipid, ceramide, and cholesterol)
Control of cholesterol homeostasis (cholesterol sensor and synthesis)
Synthesis of proteins on membrane bound ribosomes
Co-translational folding of proteins and early post-translational modifications
Quality Control (Is a protein folded properly?)
Describe co-translational translocation.
As a protein is being transcribed by the ribosome, a signal sequence is translated that directs the protein to associate with the ER. The signal receptor particle binds to this area, stops translation of the protein until the srp binds to the srp receptor in the ER, then it can continue. Ribosome can get attached to the translocon, which opens. The protein is threaded through the translocon channel and ends up properly folded in the ER.
List the major functions of the Golgi.
- Synthesis of complex sphingolipids from the ceramide backbone
- Additional post-translational modifications of proteins and lipids. For example
- Proteolytic processing
- Sorting of proteins and lipids for post-Golgi compartments
Name 3 well studied vesicle coats and describe how these coats function in vesicular transport.
The three well-studied coat structures (clathrin, COPI and COPII) The coats are assembled at sites of vesicle formation from soluble cytoplasmic components and sort the molecules (proteins and lipids) to be moved forward or backward
Coat formation assists in physical deformation of the planar membrane
Where do COP1, COP2, and clathrin take things to/from?
Cop 1: Golgi to ER
Cop 2: from ER to Golgi
Clathrin: from Golgi to membrane, membrane to lysosome, all around.
What is the structure of the nuclear pore complex and roles of nucleoporins?
Structural components include a plug and spoke complex with 8 fold symmetry. Asymmetrically distributed in nuclear membrane; synthesized throughout cell cycle. Acts as a selective filter so that certain things (water, sugar, etc) can pass through. Capable of bidirectional transport. Max size of 39nm diameter for transported particles.
What are the roles of karyopherins in nuclear import and export?
The binding receptors are part of the karyopherhin family. They directly interact with the FG Nups or with the cargo itself. Are importins or transportins. Interacts with the nuclear pore complex. You need the receptor to recognize specific cargo molecules for their entry into the nucleus if it isn’t something that always should be transported inside. Helps regulate nuclear entry. Requires Ran binding.
What is the role of Ran gradients in establishing the directionality of transport?
There is an energy cost for establishing the gradient.RanGTP is required for export and for separating the cargo from the receptor upon import. Perhaps high concentrations of RanGTP affect the volume of import/export.
How do changes in nuclear transport and/or the NPC lead to disease?
NP fusions occur in cancer. In acute myeloid lukemia, two NPCs fuse and interact with hox proteins, which can turn on transcription and inappropriately activate some genes. Specific nucleoporins play lots of roles in other diseases. They are also involved in diseases of aging. In post-mitotic cells, NPCs can become prone to oxidative damage and the pores can become leaky. The wrong proteins can accumulate in the nucleus.
What is the role of post-translational modifications in regulating nuclear import and export?
The NPC’s can form chromatin loops and wait for HAT’s to come make them ready to be transcribed
Describe two major routes for small volume endocytosis.
1) Clathrin coated vesicles: The coat forms, get pinched off by dynamin, and then gets uncoated to show the vesicle. 2) Caveolae: Surface of cell has lots of small invaginations called caveolae. They look like little caves. There are 3 major coat proteins called caveolins (1,2,3). These proteins form around the invaginations. They help form the cave like structure in the membrane. They also associate with other proteins that dictate what can come inside. Also pinched off by dynamin. Does not take things to lysosomes, which is sweet if you are a virus.
Why do proteins need to be degraded?
Misfolded proteins need to be trashed. Proteins don’t live forever. Organelles get damaged and turn over. Both good and bad endocytosed materials need to be degraded.
How is quality control of protein synthesis ensured in the ER?
The ER provides an optimized oxidizing environment for folding and oligomeric assembly
Folding ensymes (ERp57- thiol oxidoreductase allows formation of disulfide bonds)
Molecular Chaperones- ATPases (BiP- Hsp70 family)
Folding Sensors / Quality Control (UDP-glucose: glycoprotein glucosyltransferase (UGGT))
Describe two types of molecular chaperones.
Molecular chaperones help fold a protein by binding to exposed hydrophobic patches in incompletely folded proteins. Hsp70 is soluble, exists by itself, binds to proteins that aren’t folded yet. Hsp60 forms large barrel-shaped structures to act as an “isolation chamber” into which misfolded proteins are fed to prevent their aggregation and help it to refold.
