7.4-7.14 Flashcards
SRP
ribonucleoprotein particle
composition of SRP
- ribonucleoprotein particle - made of 6 polypeptides and small 7S RNA molecule
part of SRP has ____________ activity
GTPase
true or false: SRP is a G protein
true
the part of SRP that binds to the nascent polypeptide’s signal sequence has a large number of what?
Met residues (hydrophobic)
part of SRP binds to where?
the ribosome
when does SRP slow tsl until
docking at the ER
why does the SRP slow tsl
to prevent outpacing available machinery
SRP receptor (SR) composition
dimer of two subunits
2 subunits of SR
- SRa (c-face, interacts with SRP)
- SRB (transmembrane)
SRa and SRB are _____-binding proteins
GTP (GTPases)
what is required by SRP and SR for proper targeting of nascent chains to the ER and for transfer to the translocation channel and recycling of SRP to the cytosol?
coordinated GTP binding and hydrolysis
when SRP releases the ribosome, what happens?
ribosome engages the translocon and nascent polypeptide begins translocation
transport of the polypeptide into the ER lumen is through what?
an aqueous channel
translocon
comprised of the channel + other proteins closely associated with it
true or false: ions can cross the membrane while a polypeptide is being translocated through the channel
false
after the polypeptide is released and the ribosome is still attached, can ions permeate through the channel?
yes
Sec61
part of the translocon that actually forms the channel through which the translocating protein passes
composition of Sec61
heterotrimeric complex shaped like an hourglass in a cross section
- central pore occluded by small plug
what commits the chain to translocation?
recognition and insertion of signal sequence
what is probably displaced as translocation begins?
channel plug
how are some loops formed during translocation
some proteins able to transiently slip out of the gap between the ribosome and the translocon to create a loop
post-translational translocation
tsl proteins completely in the cytosol, keep them unfolded, and then translocate them into the ER
what is post-translational translocation independent of?
SRP or SR - use other Sec proteins instead
true or false: the protein is still associated with the ribosome during post-translational translocation
false
what prevent folding of protein during post-translational translocation
chaperoning of the hsp70 family
what do all chaperonins require
ATP
what is the channel for post-translational translocation
Sec61
what is most important for recognition of a signal sequence by a channel
hydrophobicity of the signal
what seems to be the main energy source driving post-translational translocation?
ATP hydrolysis by the ER-lumenal hsp70 BiP or SecA
role of BiP in post-translational translocation
ATP hydrolysis causes conformational change in BiP, causing polypeptide to be actively pulled through the channel (euks)
role of SecA in post-translational translocation
ATP hydrolysis also causes conformational change, and pushes protein through (prokaryotes)
how do some proteins become integrated in the membrane
hydrophobic anchor sequences recognized as they emerge from the ribosome, then are transferred out of the translocon into the bilyaer
true or false: a signal/anchor sequence can only be used for recognition
false, serve as dual-purpose signal and anchor sequences
when is the N-terminal signal likely cleaved
shortly after the beginning of translocation
how do signal anchor proteins target?
by using an internal transmembrane domain (the signal anchor)
2 orientations of proteins being translocated
- N-terminal translocated into ER lumen
- C-terminus translocated into ER lumen
what dictates whether the N or C terminus will enter the ER lumen first
distribution of charged residues on either side of the transmembrane domain
polytopic proteins
proteins that span the membrane multiple times
how are polytonic proteins integrated
one at-a-time or in pairs
if the ER-signal is _____________, it is almost always cleaved after it has served its purpose
N-terminal
signal peptidase complex composition
5 subunits: 2 proteolytic, 3 regulatory
what is the N-terminal cleavage site influenced by
aa residues in immediate vicinity
what is the signal peptide processed by after its removal?
signal peptide peptidase
- cuts it into pieces
