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Membranesdividethecytoplasmofeukaryoticcellsintodistinctcompartments.
Threecategoriesineukaryoticcells:
(1)theendomembranesystem:ER,Golgicomplex,Lys.,
secretoryvesicles.
(2)thecytosol.
(3)mitochondria,chloroplasts,peroxisomes,andthenucleus.
Membrane-boundstructures(organelles)arefoundinalleukaryoticcells.
Insightsgainedfromautoradiography;
Insightsgainedfromthebiochemicalanalysisofsubcellularfractions;
Insightsgainedfromthestudyofgeneticmutants;
Thedynamicactivitiesofendomembranesystemsarehighlyconserveddespitethestructuraldiversityofdifferentcelltypes.
DeDuve,,1974NobelPlrize
Reticulum(ER)
RoughendoplasmicreticulumandSmoothendoplasmicreticulum
RERhasribosomesonthecytosolicsideofcontinuous,flattenedsacs(cisternae);SERisaninterconnectingnetworkoftubularmembraneelements.
Microsome(100-200nm)
rERofpancreaticcells
Microsomesareheterogeneousmixturesofsimilar-sizedvesicles,,allowingstudiesoffunctionandcomposition.
ProteinssynthesizedonribosomesofrERinclude:
secretoryproteins,
integralmembraneproteins,
solubleproteinsoforganelles.
Modificationandprocessingofnewlysynthesizedproteins:glycosylationintherER;
N-linked:linkedtotheamidenitrogenofasparagine(ER)
O-linked:linkedtothehydroxylgroupserineorthreonineviaGalNac(inGolgi)
Theprecursorof14residuesisthesameinplants,animals,andsingle-celledeukaryotes
thenremove3glucosesand1mannoseintheER
Qualitycontrolofofnewlysynthesizedproteins---TheroleofN-linkedglycosylationinERproteinfolding
Qualitycontrol:ensuringthatmisfoldedproteinsdonotleaveER
ThelumenofrERcontains:
Bipandcalnexin(chaperones):thatrecognizeandbindtounfoldedormisfoldedproteinsandgivethemcorrectconformation;
Proteindisulfideisomerase(PDI);
GT(glucosyl-transferase,monitoringenenzyme)recognizeunfoldedormisfoldedproteinsandaddsaglucosetotheendofoligo..
Synthesisofmembranelipids
MostmembranelipidsaresynthesizedenterlywithintheER.
Therearetwoexceptions:
sphingomyelinandglycolipids,(beginsinER;completedinGolgi);
(2)someoftheuniquelipidsoftheMitandChlmembranes(themself).
Themembranesofdifferent0rganelleshavemarkedlydifferentlipidscomposition.
Transportbybudding:ER→GC、Ly、PM
Transportbyphospholipidexchangeproteins(PEP):ER→otherorganelles(includingMitandChl)