Chapter 15 Flashcards
What is the purpose of the endocytic pathway? Where does it start and where does it end?
The endocytic pathway is responsible for the ingestion and degradation of extracellular molecules. It moves materials from the plasma membrane through endosomes to lysosomes.
Define Receptor Mediated Endocytosis
Special type of endocytosis in which receptor proteins on the cell surface are used to capture a specific target molecule
What do low density lipoproteins (LDL) do?
*Might not need to know
They travel with molecules like cholesterol because they are not soluble
What does clathrin do? (general) What is its structure?
Forms a protein coat with vesicles traveling between membranes, three-legged or triskeleton
Where is clathrin used? What partner does it require?
From the golgi on the outward secretory pathway and from the plasma membrane on the inner, GTPase (makes vesicle release cargo)
What does clathrin form at the plasma membrane? Why does it hang out around the plasma membrane?
Coated pits, it is involved in endocytosis
What is the purpose of protein coats? How many types are there (general-> 1, 3, lots)
Help shape membranes into buds and help capture distinct/specific molecules
Protein coats differ depending on the origin of the vesicle
What is the role of adaptin in endocytosis?
It binds to cargo receptors on the plasma membrane to take up molecules
What is the role of dynamin in endocytosis?
It circles the end of the vesicle bud and helps separate it from the plasma membrane
What are the three possible fates for receptors used in endocytosis?
Recycled from the endosome back to the plasma membrane by budding and fusion of vesicles
They can continue to the lysosome along with the cargo from degradation
They can be used for transport in vesicles to another region of the membrane
What do lysosomes do? How can they do their job?
They are the principal site of inner cell digestion and contain many acid hydrolases that are kept functional by the acidic environment created by proton pumps
Nucleases for nucleic acids, proteases for proteins, glycosidases for carbohydrates, lipases for lipids
What is the first step in the formation of the proteins that will become a part of a lysosome?
Since they are part of the secretory pathway, they have ER signal sequences and are inserted into the lumen of the ER
How do the proteins that will become a part of a lysosome move from where they are translated to the lysosome? (include intermediate destinations)
An oligosaccharide is added in the ER, then it moves to the cis golgi where its mannose sugar is phosphoylated, then the trans golgi where a receptor protein will bind to mannose and it is placed in a vesicle
The vesicle will fuse with the late endosome, turning it into a lysosome
What is an endosome?
A precursor to lysosome that has a lowish pH that breaks cargo-receptor bonds in endocytosis
What are the three main pathways to the lysosome?
Endocytosis: Early endosomes contain cargo from endocytosis and pH is lowered to turn it into the late endosome (needs proteins to turn into lysosome)
Autophagy: Old organelles will be packaged by autophagosomes inside of a double membrane, which fuses with a lysosome
Phagocytosis: Large molecules that the cell has taken up fuse with late endosomes
What are peroxisomes and how do they get their proteins?
Organelle containing one or more enzymes that break down hydrogen peroxide and also synthesize some phospholipids
A short import signal on their proteins is reognized by receptors in the cytosol and moved to the peroxisome by protein translocators
What are the four functions of the rough endoplasmic reticulum?
Begin glycosylation of proteins
Catalyze disulfide bond formation
Quality control of protein folding
Response to unfolded proteins
Define glycosylation
the covalent attachment of oligosaccharide chains to proteins from a membrane lipid
What is(are) the purpose(s) of glycosylation? When does it happen?
Protect the protein from degradation, act as a transport signal, and participate in cell-cell recognition
During translation
Describe the process of glycosylation (and the two types?)
How does it occur at the beginning
The oligosaccharide comes attached to a lipid in the ER membrane (usually dolichol)
Oligosaccharide transferase catalyzes the transfer of the oligosaccharide from the lipid
It attaches on a asparagine amino acid and the process is called n-linked glycosylation
It attaches on the hydroxide group of serine or threonine and is called o-linked glycosylation
What do chaperone proteins in the ER do?
Bind to proteins and prevent misfolded ones from leaving
They will help them fold correctly, or lead to protein degradation (ER associated degradation) ERAD
Describe the Unfolded Protein Response (UPR) and its possible actions
Response to large amounts of misfolded proteins that is triggered by binding to receptors
1) Produce more ER membrane
2) Increase transcription/translation of chaperone proteins
3) Increase rate of ERAD
4) Decrease protein production
5) Undergo apoptosis or programmed cell death
Where do properly folded proteins in the ER go?
golgi
What is different about the ribosomes on the RER? What do they do? How did they get to the RER?
They happen to be synthesizing proteins that are part of the secretory pathway (otherwise identical) and are going to the golgi, endosomes, lysosomes, or cell surface
They translate proteins as inserts into the RER, where glycosylation occurs immediately
Signal sequences on the protein direct the ribosome to the RER, where the sequence is bound by signal-recognition particles (SRPs) that are then bound to their own receptors and lead the protein to a translocation channel
Once in the channel, a signal peptidase cleaves the signal sequence
What is the process of inserting an ER membrane protein that crosses the membrane only once?
ER membrane proteins are pulled in and the signal peptidase cleaves the start-transfer sequence
This is the same as before, but this protein also has a hydrophobic stop-transfer sequence that stays in the translocation channel
Start-transfer -> stop-transfer
What is the process of inserting an ER membrane protein that crosses the membrane twice?
ER membrane proteins that cross twice have an internal hydrophobic start sequence and a hydrophobic stop-transfer sequence
Start-transfer sequence is caught in the channel, and the protein loops around so the stop sequence is caught in the channel (continuous loop on lumen side, two ends on cytosol side)
Internal start-transfer -> stop-transfer
What do COP-coated vesicles do?
Transfer material between the ER and cis golgi network and transfer material from the trans golgi back to the cis golgi or ER
Describe the golgi apparatus, its parts, and its location
Made of a series of flattened sacs called cisternae (in stacks from 3-20)
Cis side that faces the ER and trans side where proteins leave to go to the lysosome or PM
Often located near nucleus/centrosome
What is the purpose of the golgi apparatus? How does it do its job?
continue the glycosylation of proteins by taking the proteins that enter the cis face and add or remove sugars as it passes through via many different enzymes on the two faces
sulfation and phosphoylation of proteins
Describe the process of proteins leaving the golgi (short)
they are sorted and packaged into vesicles for secretion or transport into the lysome
Define exocytosis and describe the two types
the process by which vesicles that leave the golgi will fuse with the plasma membrane
Constitutive Secretion: Default, no signal required, automatically goes to plasma membrane
Regulated Secretion (not in all cells): Many vesicles hang out near the plasma membrane waiting for signal before cytosis, Can release mucus, hormones, or digestive enzymes
HOW WE KNOW…..
Describe the in vitro protein assays that can determine the purpose of a signal sequence
Uses a protein without a signal sequence, one with, and a isolated organelle
(proteins can be labeled with radioactive amino acids)
A) Mix all together, proteins in organelle will sink to bottom of centrifuge tube
B) Mix all together, organelle will protect labeled protein from protease, unless detergent is added
HOW WE KNOW…..
Describe the use of yeast secretory pathway mutants
Some mutations result in temperature sensitive alleles where transport proteins only work at low temp
Shifting cells to high temperature will show where the transport protein works
Will accumulate in organelle at the start of the normal transport
HOW WE KNOW…..
Describe the use of GFP fusion proteins
Can be used in conjunction with secretory pathway mutants
Will show where the accumulation is and where it goes when the temp is lowered again
What are the four functions of the smooth ER?
Lipid synthesis (membrane lipids and steroid hormones)
Storage of Ca++ (used in signaling)
Drug detoxification
Carbohydrate metabolism
What are the three general transport types?
Through nuclear pores (requires unfolding and protein translocators)
Across membranes
By vesicles
What is the endomembrane system?
The nuclear membranes and the membranes of the ER, Golgi, endosomes, and lysosomes originated from invagination of the plasma membrane
These are all members of the endomembrane system
Describe the purpose of Ran and SNARE proteins in vesicle transport
they direct vesicle interaction with target membranes
Rab binds a tethering protein
V-SNARES on vesicles bind to T-SNARES on the target membrane
SNARES facilitate/catalyze membrane fusion
What is the role of the cytoskeleton in celllar transport?
The cytoskeleton forms tracks for the vesicles
Describe the nuclear envelope
The nuclear envelope encloses the nuclear DNA and defines the nuclear compartment. It is formed by two concentric membranes, the inner membrane with proteins that act as binding sites for both chromosomes and the nuclear lamina (fibrillar network, supports envelope), and the outer membrane that is continuous with the ER membrane.
Describe the nuclear pores. What do molecules need to enter?
Nuclear pores are large, elaborate structures with many proteins and symmetry. Many have unstructured regions of disordered polypeptide chains that form a mesh that stops large molecules while allowing in small, soluble ones. Large molecules must have a sorting signal to enter, the nuclear localization signal (NLS).
Describe the process of the importation of large molecules into the nucleus via the nuclear pores. Include names of helper proteins and their end fate
The nuclear import signal (NLS) is bound by a nuclear import receptor (importin)
The cytosolic fibrils then help the protein/receptor through the network of nuclear fibrils
Ran and GTP binds to the receptor, causing it to release its cargo
Back out in the cytosol, Ran hydrolyzes its GTP and dissociates from receptor
Describe how proteins are imported into mitochondria and chloroplasts
Imported proteins must have their signal sequence (mitochondria makes some of its own)
They are transported simultaneously across both membranes
As they are transported, they are unfolded and their signal sequence removed
Chaperone proteins help the protein across and fold it when it is inside
Are signal sequences on the N-terminus or C-terminus
N-terminus
