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
