Class Notes (1,100,000)
CA (630,000)
McGill (30,000)
Biology (2,000)
BIOL 201 (200)
Lecture 20

BIOL 201 Lecture Notes - Lecture 20: Hydrolysis, Cytosol, Copi


Department
Biology
Course Code
BIOL 201
Professor
Gary Brouhard
Lecture
20

This preview shows page 1. to view the full 4 pages of the document.
When%a%vesicle% gets%released%for%secretion,%it%has%to%know%whether%the%vesicle%is%
going%out%or%coming%in.
Roles%of%Secretion
Cell%Growth%-->%plasma%membrane%gets%larger%due%to%a%process%of%secretion.
Blocking%secretion%blocks%the%ability%for%cells%to%grow.
1.
Synaptic%Transmission
Neurotransmitter%release%is%a%process%of%secretion%&%is%fundamentally%the%
same%to%what%happens%in%yeast%organisms.
2.
Maintaining%metabolism
3.
Golgi%Apparatus
Structure%consists%of%packed%membranes%with%secretory%vesicles%budding%in%&%
out%of%them.
Yeast%experiment%by%Shekman%allowed%for%the%discovery%of%23%mutants%with%
defective%secretion.
Secretory%vesicles%in%yeast%provide%plasma%membrane%for%growth%&%
secrete%digestive%enzymes%like%invertase.
He%couldn’t%isolate%a%secretion%mutant%because%it%would%die.
So,%he%set%up%a%temperature%sensitive%screen%that%looked%for%mutants%in%
yeast%that%only%failed%in%secretion%when%shifted%to%the%restrictive%
temperature.
Grow%mutants%at%25,%then%shift%to%37%&%looked%to%see%if%mutant%died%and%
protein%was%defective.
Would%see%that%sucrose%was%no%longer%being%broken%down%in%
mutant%yeasts.
§
Secretion%process%failed%at%different%parts%of%the%process.
§
Sec mutants%fell%into%distinct%categories:
Failure(of(ER(import -->%protein%can%be%made/translated,%but%doesn't%
make%it%inside%the%ER%so%there's%no%secretion.
1.
Failure(to(produce(ER(vesicles -->%proteins%can%make%it%into%the%ER,%but%
they%can't%make%it%into%ER%produced%vesicles.%This%results%in%the%reduction%
of%the%Golgi%apparatus%and%the%swelling%of%the%ER.
2.
Failure(of(ER(vesicles(to(fuse(with(the(Golgi -->%numerous%vesicles%from%
the%ER%pile%up%around%the%Golgi,%but%can't%fuse%inside%them.
3.
Failure(to(produce(Golgi(vesicles -->%vesicles%can%fuse%to%the%Golgi,%but%
they%can't%be%released%from%it%which%results%in%enlarged%Golgi.
4.
Failure(of(Golgi(vesicles(to(release(-->Golgi%can%take%in%and%produce%
vesicles%&%secrete%them,%but%they%pile%up%in%the%cytosol%because%they%can't%
be%released%from%the%plasma%membrane.
5.
Factors%that%Rothman%identified%were%the%protein%products%of%the%genes%
Schekman%mutagenized.
Vesicles
Golgi%vesicles%need%to%have%a%coat%to%make%&%identify%them,%but%before%fusion%
the%coat%needs%to%be%removed%because%the%coat%proteins%need%to%get%out%of%the%
way.
When%a%coat%dissociates,%a%tag%should%be%kept%no%matter%what%because%
you%still%need%to%have%a%way%of%identifying%the%vesicles%&%what%to%do%with%
the%contents%of%the%vesicle.
The%coat%proteins%%specify%the%identity%and%shape%of%vesicles%&%are%a%
complex%of%many%different%proteins.
Coat%have%a%curvature%to%them%&%BAR%domains%set%the%radius%of%the%
vesicle.
§
There%are%different%coats%for%different%purposes.
Coats%will%differ%if%transport%is%outward%(anterograde),%or%if%transport%is%
inward%(retrograde).
COPII(coats are%for%vesicles%that%need%to%be%transported%outward;%from%ER%
to%the%cis-Golgi%network.
COPI(coats are%for%transporting%vesicles%inwards/backwards;%from%the%cis-
Golgi%to%the%rough%ER.
Clathrin(coats are%for%transporting%endocytic%vesicles%into%the%cell% from%
the%plasma%membrane.
The%something%about%the%content%of%the%vesicle%targets%a%specific%coat.
Signal%sequences%on%the%cargo%proteins%bind%to%membrane%cargo-receptor%
proteins%which%tells%the%cell%which%type%of%coat%protein%is%needed%for%the%
vesicle%in%terms%of%its%content.
These%are%not%the%same%signal%sequences%that%target%a%polypeptide%
sequence%to%the%ER%during%translation.
A%protein%needed%by%the%plasma%membrane%will%need%a%signal%sequence%.
ARF(family(proteins are%GTPases%that%control%vesicle% coat%assembly.
Sar1%which%starts%out%in%the%GDP%state%is%recruited%into%the%ER%membrane%
by%a%secretion%mutant,%Sec12.
When%the%ARF%protein%binds%to%the%sec%membrane%protein,%the%GDP%gets%
kicked%out%and%an%GTP%binds.
In%the%GTP%state%it%inserts%the%helix/hydrophobic%N-terminus%into%the%ER%
membrane.
This%event%triggers%the%event%of%onset%vesicle%formation%because%it%
can%now%recruit%coat%proteins%that%bind%to%the%ARF%and%content%of%
the%vesicle%pulling%it%off%the%ER%(creating%a%bud).
§
The%GTP%hydrolysis%of%ARF%proteins%can%trigger%coat%disassembly.
Causes%the%ARF%to%retract%its%hydrophobic%helix%from%the%ER%
membrane%and%bind%it%back%to%the%ARF%protein.
§
Causes%the%affinity%for%the%ARF%proteins%for%the%coat%proteins%to%fall%
&%the%coat%will%now%disassemble.
§
When%the%coat%disassembles%from%letting%go%of%the%vesicle,%it%can%go%
back%down%to%form%around%another%vesicle.
§
Rate%constant%of%GTP%hydrolysis%determines%how%long%the%vesicles%get%held%
up%by%the%coat%before%letting%go.
Now,%we%have%an%uncoated%vesicle,%but%we%still%need%to%be%able%to%identify%it%to%
know%where%it's%headed%in%the%cell.
Rab(proteins(which%are%GTPases%maintain%the%identity%of%uncoated%
vesicles.
Each%layer%of%the%Golgi%apparatus%is%identified%by%a%different%Rab%protein.
COPII%coats%pull%cargo%out%of%the%ER%in%vesicle%form%&%release%it%
which%then%fuses%to%the%Golgi%at%the%bottom%of%the%stack.
§
Each%presence%of%specific%Rab%proteins%will%identify%how%far%up%the%
stacks%you%have%moved.
§
The%fusion%event%to%the%vesicle's%targeted%membrane%is%mediated%by%pairs%of%
SNARE(proteins.
V-SNARE%proteins%in%the%vesicle%membrane%will%find%a%matching%t-SNARE%
protein%&%will%form%a%SNARE(complex.
This%event%docks%the%vesicle%at%its%target%membrane.
Oligomerization%of%complex%(coils)%is%responsible%for%triggering%the%fusion%
event%of%the%vesicle%to%its%target.%
NSF%complex%unwinds%the%coiled%SNARE%complex%releasing%it%back%into%the%
membrane.
Vesicles%are%transported%by%motor%proteins%like%kinesins%and%dyneins.
Adaptor%proteins%that%recruit%these%motor%proteins%to%vesicles%interact%
with%Rab%proteins%on%vesicles.
Microtubules%do%not%always%reach%the%plasma%membrane%because%of%the%cell%
cortex%(actin%cage).
Therefore,%there%are%myosins%that%function%similarly% to%kinesins%to%allow%
transport%of%vesicles%to%plasma%membrane.
Myiosin-5%is%a%two-footed%processive%myosin%that%can%do%the%same%
walking%mechanism%as%kinesin.
When%the%vesicle%reaches%the%cell% cortex,%it's%released%from%microtubles%
and%the%actin%myosin-5%carries%it%out%the%rest%of%the%way%past%the%actin%cell%
cortex.
Secretion)and)Vesicle)Transport
11:04%AM
You're Reading a Preview

Unlock to view full version