7.1-7.5 updated Flashcards
what do cells depend on to ensure proteins arrive at proper sub cellular destination
signals on proteins
proteins with no targeting signal will be tsl entirely where?
on free ribosomes and will remain in the cytosol
what organelles receive proteins translated entirely in the cytosol on free ribosomes?
mitochondria, chloroplasts, nucleus, peroxisomes
what organelles are part of the secretory pathway?
ER, Golgi, lysosomes, endosomes, plasma membrane
how are proteins transferred in the secretory pathway
budding and fusing
steps of protein targeting and entering the secretory pathway
- signal sequence binds to a signal recognition particle which causes tsl to temporarily halt
- ribosomes, nascent polypeptide, and SRP arrive at the ER, interact with a receptor
- channel opens, nascent protein beings to be co-translationally translocated
who created the signal hypothesis
Gunter Blobel (1970s)
what does the signal hypothesis explain
how proteins got targeted to the ER
when is the ER-targeting signal usually cut off
inside the RER after the protein begins its translocation
term for a nascent protein prior to its signal sequence removal
pre-protein
experimental evidence of signal hypothesis
- in vitro w/o microsomes: larger protein, migrates slowly
- in vitro w/ microsomes; protein smaller and migrated faster, matched protein located in ER lumen
SRP
ribonucleoprotein particle
- made of 6 polypeptides and a small 7S RNA molecule
where is SRP found
cytosol
part of SRP has ________ activity
GTPase - it’s a G protein
part of SRP that binds to the nascent polypeptide’s signal sequence
large number of Met residues and is hydrophobic
true or false: proteins in the secretory pathway are always synthesize on bound ribosomes
false: can start on free ribosome then be co-transitionally translocated or can have posttranlational translocation
what happens when part of SRP binds to the nascent protein
tsl halts until docking at ER
what is SRP receptor composed of?
dimer of two subunits: SRa and SRB
true or false: each subunit of the SRP receptor is considered a monomeric GTPase
true
SRa
on c-face, interacts with SRP
SRB
transmembrane
SRa and SRB are also _______________
GTP-binding proteins (GTPases)
the GTP is required for what step of SRP/receptor interactions?
releasing
what happens when SRP releases the ribosome
ribosomes engages translocon and nascent polypeptide begins translocation
2 big categories of G proteins
- heterotrimeric (GPCR binding, aBy)
- monomeric (SRP, SRa, SRB)
characteristic cs of G proteins
- all GTPases
- all GTP-binding proteins
- all able to perform their unique function only when GTP-bound
