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
