Lecture 13 - Organelles 3 Flashcards
What is exocytosis?
Process by which newly made proteins and lipids are delivered from the ER, via the golgi, to the cell surface and cell exterior by transport vesicles that fuse with the plasma membrane
What are the functions of the golgi?
- Sorting and dispatching station for proteins and lipids made in ER
- modification of N-linked oligosaccharide chains on glycoproteins made in ER
- Synthesis of O-linked oligosacchardis on proteins and lipids made in ER
- Synthesis of glycosaminoglycan chains on core proteins of proteoglycans
Describe the structure of the golgi
Flattened membrane-bounded cisternae stacked together
Two distinct faces: cis (entrey face, adjacent to ER), trans (exit, pointing toward plasma membrane)
network of interconnected tubular and cisternal structures
Usually located near the nucleus
of golgi stacks varies per cell
Describe the process of protein sorting.
Proteins get sent through cis then trans networks and can be sent 3 ways - to the lysosome, plasma membrane, and secretory vesicle
The pathway to the plasma membrane is the DEFAULT
Describe the modification of N-linked oligosacchardies in the Golgi
An ordered series of series of sugar removals and additions carried out by glycosideases (remove sugars) and glycosyl transferases (add sugars). There is a varying amount of modification but the High Mannose form is the least modified, the “Complex” is the most modified” and the hybrid is in between the two.
Describe O-linked glycosylation in the Golgi
Covalent attachment of oligosaccharides to the OH groups of serine and threonine in proteins (o linked). Differs from synthesis of N-linked in the ER by:
Sugars are added to the proteins one at a time
Sugars are added post-translationally (by glycosyl transferases)
Describe the difference between proteoglycans and glycoproteins
Proteoglycans = small core protein, >95% carbohydrate by mass, carbs are made out of glycosaminoglycan chains made of repeating disaccharide units (6 classes of them)
Attract water to form lubricants and gels that spring back (synovial fluid)
Found in ECM and on cell surfaces
Glycoproteins - any protein with one or more covalently bound carb units that do not contain a serial repeat unti (aka no glycosaminoglycans), typically (but not always) are protein w/ little card
Describe the synthesis of proteoglycans in the Golgi
Core protein synthesized in the ER > transported to the golgi
In the golgi, glycosyl transferases act sequentially to build the 4 sugar linker region on a serine of the core protein
Repeated action of the two specific glycosyl transferases adds repeating sugars to the chain
Addt’l enzymes may modify sugars (sulfation)
Core protein can also contain N and O linked sugars
How are glycoproteins and proteoglycans degraded?
Occurs in lysosomes by hydrolases…
Endoglycosidases first remove carbohydrates from proteins
Proteases cleave protein component into amino acids which can be resued
Glycosidases act on each glycosidic bond in reverse order in which they were built (last on, first off)
What is the function of glycoproteins?
Most of the soluble and membrane proteins synthesized in the ER, including those destined for other locations, are glycoproteins.
In contrast, very few cytosolic proteins are glycosylated.
Have a ton of functions, including…. ABO blood groups, collagen, FSH, mucins, proteases,
What is the glycocalyx?
The thick layer of carbohydrate on the cell surface - help mediate wide variety of functions at the cell surface
Composed of glycolipid, glycoproteins, and proteoglycans!
Describe mucins
The most abundant macromolecules in mucus are mucins - synthesized and secreted by specialized cells within the epithelium.
Mucins are vicous glycoproteins (~80% carb by mass), most are O-linked. Often polymerize to generate hydrated gel and resistant to proteases.
Discuss the structures and important of ABO blood group antigens
They are oligosaccharide components of glycoproteins and glycolipids on surface of RBCs (extremely immungenic and important in transfusions)
The ABO gene encodes a glycosyl transferase and determines if a sugar will be added to O antigen
O antigen - encodes non-functional protein
A antigen - encodes transferase that transfers N-acetyl galactosamine (GalNAc)
B antigen - encodes transferase that transfers galactose
A and B are codominant, O is recessive
Describe how types O, A, B, and AB connect to each other
Type O - they are universal donors since they only contain O antigen, no A or B to be recognized by recipient’s immune system
Type A (AA or AO) - make GalNAc transferase, have A antigen, can only donate to A and AB. Only can accept A
Type B (BB or BO) make Gal transferase, B antigen, can only donate to B or AB. Can only receive B
Type AB - universal acceptor, have both antigens (no antibodies)
Describe the constitutive exocytosis pathway
Called the “default” pathway - all proteins get sent through this by the golgi unless they have a specific signal
Vesicles bud from trans golgi and fuse w/ plasma membrane
Operates continually in all cells
Describe regulated exocytosis pathway
Found in specialized secretory cells (those that secrete hormones or digestive enzymes)
Proteins are diverted into secretory vesicles which bud off from trans golgi and accumulate near plasma membrane
Vesicles fuse w/ plasma membrane to release contents ONLY in response to an extracellular signal
What are the endocytic pathways?
Pinocytosis - uptake of fluid and small molecules in small vesicles (mediated by clathrin coats)
Continuous process in all eukaryotic cells
Phagocytosis - uptake of large particles such as bacteria in large vesicles (phagosomes)
requires receptor activation at cell surface
occurs in specialized cells (macrophages and neutrophils) - fuse w/ lysosomes and ingested material is degraded
Discuss receptor-mediated endocytosis and an example of when this doesn’t work..
Classic example is of the uptake of low density lipoprotein (LDL)
Most cholesterol transported as LDL in blood
When cholesterol is needed for membrane synth, cells make LDL receptors and insert them into membrane
LDL receptors taken into cell via clathrin coated pit w/ LDL attached
Vesicles shed clathrin coat and fuse w/ endosomes (acidic, cause dissociation of LDL from receptor)
LDL transported to lysosomes and hydrolyzed to free cholesterol
LDL receptor is recycled back to cell surface
When LDL receptors are defective, cholesterol uptake is blocked and accumulates in blood
Lots of heart attacks, plaque everywhere
What are early endosomes?
Located near the plasma membrane, act as the main sorting station in the endocytic pathway - acidic environment allows some receptors to release their ligands, ligands normally taken to lysosomes
Describe late endosomes
Located near nucleus - endocytosed materials arrive here 5-15 minutes after uptake
Materials are ultiamtely transported to lysosoms or late endosome is converted into lysosome by import of lysosome proteins
What are the options for receptors in early endosomes?
- Recycled back to plasma membrane
- Degradation - lysosome breaks that shit down ( decreases [receptor], which is known as receptor down-regulation
- Transcytosis - return to a different plasma membrane domain in polarized cells (ex. mother’s antibodies are transported like this to breast milk for the baby)
Describe the structure and function of lysosomes
Principal sites of intracellular digestion
Contain ~40 hydrolases
Membranes contain ATP-driven proton pump that maintains the acidic pH (protects cell in case lysosome burst, the enzymes wouldn’t work at 7.2(
Membranes contain transport proteins - move digestion productions (amino acids, sugars, etc.) to cyotosol
Membrane proteins are heavily glycosylated to protect them
How are lysosomal enzymes delivered to the lysosomes?
Mannose-6-phosphate is the tag that’s added to lysosomal enzymes in the ER/golgi
sorted into transport vesicles in trans golgi nextwork
What are the 3 pathways to lysosomal degradation?
- Endocytosis (ex. LDL)
- Phagocytosis (ex. ingestion of bacteria)
- autophagy - digestion of obsolete cell parts - double membrane surrounds organelle forming autophagosome
Describe some of the lysosomal storage diseases
- Mucopolysaccharidoses - defects in lysosomal enzymes required for degradation of glycosaminoglycans
- Oligosaccharidoses - “” oligosaccharides
- Sphingolipidoses - “” sphingolipids (ex. Tay-Sachs)
- Inclusion-cell disease - defect in enzyme responsible for generating M6P tag on hydrolases
hydrolases fail to target to lysosomes, so they’re secreted from the cell
Accumulation of many undigested substrates forming inclusion bodies (empty lysosomes)
