LEC36: Transport into Mitochondria, Peroxisomes, & the Nucleus Flashcards Preview

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Flashcards in LEC36: Transport into Mitochondria, Peroxisomes, & the Nucleus Deck (62):

what % of proteins go through the secretory pathway versus stay in the cytosol and are targeted elsewhere? where else might they be targeted? 

30% of proteins are handled by secretory pathway at the ER 

70% stay in cytosol or are targeted to mitochondria, nucleus, or peroxisomes 


what % of liver volume do mitochondria comprise?



size of mitochondria?

0.5-1 micron diameter


function of mitochondria?

generate ATP to be used as cellular source of energy


important structural features of mitochondria?

outer mitochondrial membrane, inner mitochondrial membrane with cristae forming big folds, membrane space in between the 2 membranes, mitochondrial matrix in middle of it all



structure(s) of outer mitochondrial membrane?

porous to molecules up to 5-10 kDa

contains a protein translocation apparatus, TOM 



structure(s) of inner mitochondrial membrane?

inner membrane is 70% protein, is impermeable even to protons 

inner membrane is highly invaginated into cristae to increase surface area (cristae are variable)

has TIM complex for transport of peptides across it

membrane potential across inner membrane 


what is within the inner membrane, re: proteins?

protein complexes of the electron transport chain and ATP synthase complex that catalyzes formation of ATP from ADP 


what creates membrane potential across inner membrane?

delta psi 

matrix (inner) is negatively charged; inter membrane space (out) is highly positively charged 

protons moving into the intermembrane space creates membrane potential 

thus inner membrane is sealed tight to protons 

makes it hard to get proteins into the mitochodria 


what's different between mitochondria in the skeletal muscle vs. liver?

muscles have lots of cristae; liver do not have much cristae 

b/c liver is more involved in metabolism, and muscle is dedicated to ATP production 


what to mitochondrion do in sperm?

in flagella - so sperm can swim


what is lodged w/in the inner mitochondria membrane in terms of proteins and molecules?

channel proteins for translocation of metabolites:

pyruvate, malate, acyl-CoA, amino acids 


ADP/ATP transporter 



what is in the intermembrane space? 

between the 2 membranes 

contains enzymes that phosphorylate other nucleotides apart from ADP, eg nucleoside diphosphate kinase, which converts GDP -> GTP


what is in the mitochondrial matrix?

1) hundreds of enzymes, including those for oxidation of pyruvate, fatty acids, ketone bodies to acetyl-CoA 

2) enzymes that catalyze amino acid oxidation 

3) enzymes of tricarboxyic acid cycle and urea cycle 

4) mitochondrial genome, ribosomes, tRNAs, molecular chaperones for folding newly synthesized & newly imported proteins


what is the structure of the mitochondrial genome?


16,589 bp 

encodes 13 proteins

2-50 copies of genome per mitochondria, so up to 1000/cell


what does mitocondrial genome encode?

tRNAs and rRNAs for its own ribosomes

mitochondrial genes are txn into mRNA and tln in matrix on mitochondrial ribosomes


what proteins does the mitochondrial genome encode?

subunits for several components of respiratory chain, including cytochrome c oxidase, NADH dehydrogenase, apocytochrome b 

genes for protein synthesis within matrix: 22 tRNAs, 12S and 16S rRNA for mitochondrial ribosomes


can gene migration occur btwn the universal and mitochondrial genomes?


b/c mitochondrial genome has different genetic code than universal genetic code 

i.e. UGA is universal STOP codon; it is Tryptophan in mitochondria 

AGG is R in universal code, STOP in mitochondrial genome


how do mitochondria come to exist?

they aren't assembled; divide into 2 from 1 existing mitochondria


how are mitochondria inherited in mammals?



what is evidence of mitochondria's prokaryotic origin?

endosymbiotic origin from prokaryotes, like chloroplasts in plants 

evidence: prokaryotic character of irbosomes, molecular chaperones, and circular genome 


where do most mitochondrial proteins come from?

synthesized on cytosolic ribosomes before targeting to outer membraen and post-translational import 


what is the nature of mitochondrial targeting sequence?

15-35 residuce N-terminal sequence of positive, basic amino acids 

it is cleaved in matrix by an endoprotease 

also has a non-cleaved internal targeting sequence


when do proteins destined for mitochondria get transported?

what is the state of the peptide?


unfolded state 


what brings peptide to mitochondria? what does it do?

Hsp70 chaperone 

important b/c protein is imported in unfolded conformation, which Hsp70 maintains 


describe how peptide enters mitochondria once it is chaperoned there from the ribosome 

1) Hsp-70 bound peptide binds to receptor on outer mitochondrial membrane; is presented to TOM complex; enter through aqueous TOM40 translocation channel 

no energy involved 

2) positively charged pre-sequence of peptide opens TIM complex on inner mitochondrial membrane

electrophoretic effect occurs, b/c targeting sequence is positive compared to inner part of matrix

TIM opens enough for peptide to get in through inner mitochondrial membrane 

3) once in matrix, mitochondrial version of Hsp70 binds protein 

4) Hsp70 pulls protein into matrix, helps fold it


what are all of Hsp70's functions?

1) keep protein unfolded in cytosol 

2) pull protein in to mitochondrial matrix after presequence has entered through TIM complex of inner mitochondrial membrane 

3) helps protein fold after it has entered matrix


when does the mitochondrial targeting sequence get cleaved?

once peptide is within the matrix


what happens if have a mutation in mitochondrial DNA?

may have genotypic mutation, but it may not express disease phenotype (although it may!) 

penetrance determines this - more copies of a mutation you have in genotype, more experiences in phenotype


what is leber's hereditary optic neuropathy (LHON) caused by?

missense mutation in subunit 4 of NADH-coQ reductase 

causes sudden onset blindness, hearing loss, mental retardation 


what causes dystonia?

NADH-coQ reductase missense mutation in subunit 4

same mutation as LHON 


what does aging due to mitochondria DNA?

rearranges mitochondrial DNA 

probably result of accumulation of reactive oxygen species with time 

humans over age 40 have increased mitochondrial DNA rearrangements compared to humans under age 40 

leads to decreased energy production w/ age 


what is the structure of peroxisomes?

small, single membrane organelle 



what do peroxisomes do?

fatty acid B-oxidation aka they oxidize long chain fatty acids 

this generates hydrogen peroxide which catalase reduces to water and oxygen 

do plasmalogen synthesis - phospholipids found in omst eukaryotic cells, myelin membranes



what encodes peroxisomal proteins? where are they synthesized? when are they transported? how? 

nuclear genes 

synthesized on cytosolic ribosomes 

imported post-translationally in folded state into peroxisome 

2 C-terminal target sequences: PTS1, it is SKL a.as 



what does catalse do?

reduces hydrogen peroxide to water and molecular oxygen 

works in peroxisomes 



what are diseases related to peroxisomes? what are they caused by?

1) zellwegers syndrome - no import of any peroxisomal enzyme 

2) adrenoleukodystrophy (ALD) - oxidation of very long chain fatty acids is defective; ALD gene is membrane transporter for long-chain fatty acyl CoA synthase from cytosol to peroxisome matrix


what does nucleus separate?

genetic material inside, separate from cytosol 

separates txn and tln, allowing another level of regulation compared w/ prokaryotes 


what activites occur inside nucleus?


mRNA splicing 

DNA replication 

ribosome biosynthesis


when does nucleus form?

nucleus is disassembled prior to mitosis, reforms afterward


what is size of nucleus?

largest organelle in cell 

5-10 um diameter


what is membrane of nucleus like?

double lipied bilayer

inner = nuclear lamina, gives nucleus tensile strength

outer layer = rough ER membrane, they are contiguous


what/where is nuclear lamina?

lines inner surface of nuclear membrane 

meshwork of intermediate filament type proteins, lamins 



how does transport into/out of nucleus occur?

via aqueous pores, nuclear pore complexes 


what is the organization of chromatin?

nucleus houses all all genes, in 46 chromosomes

each chromosome is very long and thin; presents packing problem

DNA is packaged into chromatin which is further compacted into a 30 nm fiber

these 30 nm fiber are interphase nucleus chromatin structure 


what is the nucleolus? 

where rRNA is transcribed, synthesized

where ribosomal subunits are assembled

most prominent strucutre within nucleus


describe the chormatin of the interphase nucleus

euchromatin (10% of which is transcribed) and heterochromatin, which is highly condensed and not trnscribed 

highly condensed chromatin is in close apposition to the lamina 

euchromatin and heterochromatin define the interphase nucleus 

during mitosis, chromosomes condense into metaphse chromosomes that're more highly organized


where are the genes that encode rRNA?

in nucleolus, 200 copies of these  genes, including 5.8S, 18S, 28S on 5 different chromosomes 

all are synthesized as a 45S precursor 

these locii localize to the nucleolus 


describe process of ribosomal RNA biogenesis 

they congregate in the nucleolus of the nucleus 

ribosomal mRNA are synthesized on chromsomes all around the nucleus by RNA Pol II 

the mRNA are exported from nucleus; translated on cytplasmic ribosomes 

ribosomal proteins then reenter nucleus, are transported to nucleolus, as does 5S RNA 

there, assemble w/ rRNA 

individual 40S and 60S pre-ribosomal aprticles are then delivered to cytosol 



how many protein molecules enter nucleus / minute?

how many mRNA? 



60,000 protein 

50-250 mRNA 

1000 tRNA

all via 3,000 nuclear pore complexes


what is the nuclear localization sequence?

can exist anywhere on protein 

usually 4-8 a.as, rich in Arg and Lys, usually contains Pro



in what conformation do proteins enter nucleus?



are nuclear export signals = NLS?


nuclear export signal is distinct from nuclear localization sequence 


when/is the nuclear localization cleaved?


because proteins shuttle between nucleus & cytoplasm or have to reenter nucleus as it reforms after mitosis


how do proteins enter/exit nucleus?

through nuclear pore complexes: 8 subunits long ring 

bidirectional transport occurs through NPCs



what is structure of the NPC?

nuclear pore complex

3000-4000 per nucleus 

each composed of ~100 diff proteins, collective mass is 125 Md

has an 8 fold symmetry, contains central aqueous channel of 9 nm diameter and 15 nm length 

is several ring assemblies that occupy cytoplasmic face, inner core and nucleoplasmic face of structure 

filamens radiate out from both cytoplasmic and nucleoplasmic sides


describe process of protein entry into the nucleus via the NPC

if protein is made in cytosol, wants to go back to nucleus to ribosome 

1) protein binds to karyopherin receptor 

karyopherin enters through NPC w/ protein attached 

2) RAN-GTP causes disassembly of karyopherin complex, cargo protein is released

3) karyopherin is bound to RAN-GTP, they undergo export through NPC back to cytosol 

4) cytosolic RAN-GAP protein promotes disassembly of karyopherin:RAN complex by catalyzing GTP hydrolysis in Ran

karyopherin now free to recycle, pick up new cargo; Ran GDP can return into nucleus,  and Ran ntd exchange factor, RAN Nef, catalyzes formation of Ran GTP to start cycle again


what is Ran? what are its forms/where? relations to GAP and GEF?

a small GTPase w/ co-factors that regulate nucleotide hydrolysis (Ran GAP) and ntd exchange (Ran GEF) 

Ran GAP: cytosol, so Ran GDP

Ran GEF: nuclear, so Ran GTP



during process of secretion: 

is there a signal seuqnce? is signal sequence cleavable? co or post translational?

yes, signal sequence

yes, cleavable, 



during transport into mitochondria, 

is signal sequence cleavable? 

co or post translational?

yes cleavable 

post translatoinal


re: transport to nucleus, 

is signal sequence cleavable? co or post translational?

not cleavable 

post translational


for peroxisome, 

is signal sequence cleavable? 

co or porst translational?

not cleavable, and at C terminus

post translational

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