what is the golgi complex made of
- flattened membrane sacs called cisternae
- cis-golgi → closest to the ER (entry side)
- medial-Golgi
- trans-golgi → exit side (proteins leave for other destinations)
- each cisternae has different enzymes in it
example of enzymes found in cisternae
glycosidases and glycosyltransferases
how do vesicles move through the golgi
via cisternal maturation
what is cisternal maturation
a model that poses cisternae are dynamic containers that move from the cis to trans face, rather than static structures
how does cisternal maturation work
1) the incoming vesicles merge to form the new cis golgi
2) the old cis golgi is pushed forward to now become the medial golgi, while the medial is now the trans
3) the trans golgi becomes the trans golgi network
4) vesicles of enzymes are delivered backwards so that each new cisternae has the correct ones
what is retrogade loss of material
the vesicles of enzymes being delivered backwards so that each new cisternae has the correct enzymes
what’s the point of the golgi
post translational modifications - prepares proteins for the extracellular environment
examples of post translational modifications
- quality control (degrading proteins that aren’t ready for secretion)
- structural stability (assembling proteins)
- diversity of proteins (e.g. glycosylation)
- activation/inactivation of enzyme ability (activating enzymes only when needed to)
four main protein modifications
- folding and assembly of multi-subunit proteins
- disulfide bond formation
- glycosylation (carbohydrate modifications)
- proteolytic cleavages
what is glycosylation
addition of sugars (carbohydrates) to proteins
what is n-linked glycosylation
a sugar is attached to the nitrogen (N) atom in the side chain of asparagine in a protein
what is the code for n-linked glycosylation
tripeptide sequence: N-X-S/T
asparagine-X-sernine or threonine
how does n-linked glycosylation work
as the protein enters the ER lumen during translation, the tripeptide sequence is translated and enzymes will add on a sugar to the asparagine
what is the sugar added to asparagine
(Glu)3 (Man)9 (GluNAc)2
what is the enzyme that adds the sugar onto asparagine
oligosaccharyl transferase
describe the full process of n-linked glycolysation
1) as the protein is translated into the ER, the N-X-S/T sequence is translated
2) oligosaccharyl transferase recognises the sequence then transfers the sugar onto the asparagine residue
3) once the sugar is attached, glucosidase begins to remove the glucose residues
preglycan vs core glycan
- preglycan is the sugar with the glucose
- core glycan is the sugar without the glucose (after glucose removal)
example of something that depends on glycosylation
blood groups - determined by which sugars are attached to glycans on the surface of red blood cells
what is clathrin
a protein that coats and forms a large cage-like structure on vesicles budding from the trans-golgi network to the lysosome
what else does clathrin come with
an adaptor protein, as it can’t directly bind the signals on the cytoplasmic tails of the proteins
how does clathrin work
1) ARF binds GTP then recruits adaptor proteins
2) adaptor proteins bind cytoplasmic tails of cargo receptors
3) adaptor proteins then recruit clathrin, which form a cage
4) as clathrin accumulates, the vesicle forms a narrow neck
5) dynamin hydrolyses GTP around the neck of the vesicle, pinches off the vesicle and finally it buds off
6) ARF hydrolyses GTP and the clathrin coat + adaptor proteins are shed off
what do the clathrin-AP complexes do
- help transport proteins to the lysosome
- help transport material that is endocytosed from the plasma membrane
how was dynamin’s function discovered
dom-neg mutant of dynamin
- vesicles’ necks got longer and longer didn’t bud off when dynamin was mutated
what is a dom-neg mutation
a type of mutation where the mutant version of a protein interferes with the function of the normal protein, even when the normal protein is present
