L16 - protein sorting and trafficking Flashcards
List the different pathways for newly synthesised proteins
- cytoplasmic pathway
2. endoplasmic reticulum pathway
list the possible destinations of newly synthesised proteins and the pathway that takes them there.
mitochondria (cytoplasmic) nucleus (cytoplasmic) peroxisome (cytoplasmic or ER) cytoplasm (cytoplasmic) secretory vesicles (ER) lysosomes (ER) ER (ER)
what are the possible destinations of newly synthesised proteins via the cytoplasmic pathway
mitochondria
cytoplasm
peroxisomes
nucleus
what are the possible destinations of newly synthesised proteins via the ER pathway
peroxisomes
lysosomes
ER
secretory vesicles
what determines a proteins end destination within a cell?
internal ‘post codes’ - sequences in polypeptide which direct to certain organelles
briefly explain cytoplasmic pathway
cytoplasmic ribosome synthesises protein
folded protein released into cytoplasm and travels to nucleus/peroxisome/cytoplasm
or
unfolded protein released into cytoplasm which travels to mitochondria to be folded
what are Glycosyltransferases and where are they located
enzymes that add sugars to proteins
golgi
(blood group antigens)
what is the lysosomal post code and where is it added to the protein
mannose 6 phosphate
Golgi
what is I‐cell disease
- mutations in M6P or M6P receptor in golgi
- leads to no mannose 6 phos being added to proteins
proteins fail to make it to lysosomes and are secreted
define cis and trans golgi
cis - region of golgi that faces RER
trans - region of golgi that faces membrane
generally describe the ER pathway
proteins synthesised in ER
travel in vesicles to golgi
through golgi into vesicles
secreted/ lysosomes/ peroxisomes / membrane
what do proteins in the ER all have in common?
signal sequence at the N terminal
describe the signal sequence
n terminal
stretches of hydrophobic residues
what protein recognises signal sequences and will result in the protein being transported into ER
signal recognition protein SRP
describe the process of uptake of a protein into the ER
- signal sequence of PP (still bound to ribosome) is bound to by SRP
- ER membrane contains SRP receptor that binds to SRP
- SRP displaced and recycled
- ribosome- mRNA complex is transferred to transmembrane channel
- ribosome continues to synthesise protein, injecting the pp chain into the ER lumen
6.
what are the two outcomes for a protein entering the ER
- embedded in membrane due to more HPhobic stretches in the PP chain (transmembrane protein eg ion channel)
- ends up in ER lumen, transported into vesicles for transport / secretion
what does signal peptidase enzyme do?
cleaves the signal sequence (because it gets stuck in the channel as its Hphobic) so the pp can be injected into the lumen
what protein modification happens in ER
- folding (aided by chaperone proteins)
2. forming disulfide bonds
what protein modification occurs in golgi
- adding of ‘post codes’
2. glycosylation (forming glycoprotiens)
why is it important to have a specific lysosomal postcode?
lysosomes are acidic and contain degradative enzymes so we dont want the wrong proteins to be able to make it there easily
is the environment of the
cytoplasm
ER lumen
oxidising or reducing
cytoplasm - reducing
ER lumen - oxidising
describe the cytoplasmic pathway of proteins to mitochondria
- PP chan fully translated in cytosol and left partially unfolded by chaperones
- signal seq. recognised by receptor on M membrane
- TOM (outer membrane channel) and TIM23 (inner membrane channel) can associate which catalyses the entrance of the entire PP chain into M matrix
- signal seq cleaved
describe the signal seq for proteins destined for mitochondria
10-70 AA long
alternating Hphobic and positive residues which create amphipathic helix (one side hphobic one side +ve)
is import of proteins into
mitochondria
ER
co-translational or post-translational
M - post
ER - co
what keeps polypeptides destined for mitochondria partially unfolded?
chaperones
what happens if only TOM is present?
the protein may end up between the inner and outer membrane
what can cause a protein to become imbedded in a mitochondrial membrane
hphobic stretches of sequence
what do pores of the nucleus have that aid in protein transport?
spokes
fibrils
which way do the
spokes
fibrils
of nuclear pores point
spokes - point into nucleoplasm
fibrils - point out into cytoplasm
what is needed for a protein to be imported into the nucleus?
nuclear localisation signal (NLS)
describe NLS
short
positively charged
residues such as Lys, Arg
describe the process of importing a protein into nucleus
- protein carrying NLS is recognised by importin protein which binds to it
- complex binds to fibrils catalysing entrance
- complex is recognised by Ran (small GTPase) which displaces the protein from importin
- there is now a Ran-importin complex which is recognised by spokes
- spokes mediate the exit of the ran-importin complex
- Ran hydrolyses GTP to GDP releasing importin
describe the process of exporting a protein from nucleus
- protein has nuclear exit signal (NES)
- exportin recognises NES signal and binds to the protein
- Ran then binds to this complex
- exportin - protein - ran complex binds to spokes which mediate exit into cytoplasm
- ran hydrolyses GTP to GDP releasing protein and exportin
- exportin recognised by fibrils that reimport it into nucleus