Molecular Cell Biology
Fifth Edition
Chapter 16:
Moving Proteins into Membranes and Organelles
Copyright © 2004 by W. H. Freeman & Company
Harvey Lodish • Arnold Berk • Paul Matsudaira • Chris A. Kaiser • Monty Krieger • Matthew P. Scott •
Lawrence Zipursky • James Darnell
Live bovine endothelial cell:
Green is ER; Orange is mitochondria DNA → RNA → protein →Protein sorting → different organelles → different functions
The mechanisms or pathway of protein sorting / protein targeting ?
A typical mammalian cell: has about 10,000 different kinds of proteins
- cytosol
- a particular cell membrane, an aqueous compartment, cytosol, or to the cell surface for secretion
Protein targeting or protein sorting:
1) protein targeting to membrane or aqueous interior of intracellular organelle
2) vesicular-based protein sorting (secretory pathway) –chapter 17 Signal sequences (20-50 aa.), uptake-targeting sequences, receptors, translocation channel, unidirectional translocation
Press Release: The 1999 Nobel Prize in Physiology or Medicine
NOBELFÖRSAMLINGEN KAROLINSKA INSTITUTET THE NOBEL ASSEMBLY AT THE KAROLINSKA INSTITUTE
11 October 1999
The Nobel Assembly at Karolinska Institutethas today decided to award the Nobel Prize in Physiology or Medicine for 1999 to
Günter Blobel for the discovery that
"proteins have intrinsic signals that govern their transport and localization in the cell"
Summary
A large number of proteins carrying out essential functions are constantly being made within our cells. These proteins have to be transported either out of the cell, or to the different compartments - the organelles - within the cell. How are newly made proteins transported across the membrane surrounding the organelles, and how are they directed to their correct
location?
These questions have been answered through the work of this year’s Nobel Laureate in Physiology or Medicine, Dr Günter Blobel, a cell and molecular biologist at the Rockefeller University in
New York. Already at the beginning of the 1970s he discovered that newly synthesized proteins have an intrinsic signal that is essential for governing them to and across the membrane of the endoplasmic reticulum, one of the cell’s organelles. During the next twenty years Blobel characterized in detail the molecular mechanisms underlying these processes. He also showed that similar "address tags", or "zip codes", direct proteins to other intracellular
organelles.
Four fundamental questions:
1. What is the nature of the signal sequence, and what distinguishesit from other types of signal sequences?
2. What is the receptorfor the signal sequence?
3. What is the structure of the translocational channelthat allows transfer of proteins across the membrane bilayer?
In particular, is the channel so narrow that proteins can pass through only in an unfolded state, or will it
accommodate folded protein domains?
4. What is the source of energy that drives unidirectional transfer across the membrane?
Moving Proteins into Membranes and Organelles (protein targeting)
1. Translocation of secretory proteins across the ER membrane
2. Insertion of proteins into the ER membrane(glycoprotein to the
outside of membrane or release)
3. Protein modifications, folding, and quality controlin the ER
4. Export of bacterial proteins
5. Sorting of proteins to mitochondria and chloroplasts
6. Sorting of peroxisomal proteins
7. Sorting of nucleus proteins
16.1 Translocation of secretory proteins across the ER membrane
Signal sequence: for ER, peroxisome,
mitochondria, chloroplast Fig16.2 Electron micrograph of ribosomes attached to the rough ER
in a pancreatic acinar cell
Translocation of secretory protein across the ER membrane Secretory proteins are synthesized on ribosomes attached to ER (rough ER).
Free ribosome:
for cytosolic protein synthesis
How do we know that signal peptides are ‘necessary and sufficient’?
In vitro system can be used
In vitro translate mRNA for a mitochondrial protein
-- w/ or w/o signal peptide -- radiolabeled (e.g., with 35S) Incubate with organelle fraction Density centrifugation
Gel electrophoresis and autoradiography
How do we know protein is Inside the organelle?
-- protease/detergent treatment
Fig 16.3 Labeling experiments demonstrate that secretory proteins are located to the ER lumen shortly after synthesis
How to study of Secretory proteins are localized to the ER
lumen shortly after synthesis.
Cell + isotope-amino acid → new protein synthesis had isotope
→ homogenization
Centrifuge
如果protein 在外就會被 分解
A hydrophobic N-terminal signalsequence targets nascent secretory proteins to the ER
After synthesis secretory protein →signal sequence→ ER → modification (glycosylation…….)→vesicle transport to ……….
A 16- to 30-residue ER signal sequence (in N-terminal):
one or more positively charged adjacent to the core a continuous stretch of 6-12 hydrophobic residues(the core) but otherwise they have little in common is cleaved from the proteinwhile it is still growing on ribosome not present of signal sequence in the “mature” protein found in cells signal sequence is removed only if the microsomes are present during
protein synthesis
microsomes must be added before the first 70 or so amino acids are linked together in order for the completed secretory protein to be localized in the microsomal lumen
cotranslational translocation
Fig 16.4 Cell-free experiments demonstrate that translocation of secretory proteins into microsomes is coupled to translation
EDTA - ribosome free microsomes
No signal sequence No incorporation into
microsomes
Signal sequence Incorporation into
microsomes Microsomes must be
added before the 1st70 aa
Cotranslational translocation is initiated by two GTP-hydrolyzing proteins Cotranslational translocation: 必需一同參與
Ribosomeand microsomeinvolved; The first 40 aa (include signal sequence) into microsome from ribosome, next 30 aa in ribosome channel.
Secretory proteins are related with ER, but not with other cellular membrane. Has specificity of ER and ribosome interaction
DNA → RNA → cytosol → ER + ribosome → contranslation translocation → to ER
Cotranslational translocation is initiated by two GTP hydrolyzing proteins
The role of SRP and SRP receptor in secretory protein synthesis
First 70 aa
Not all signal sequence located at N-terminal
signal-recognition particle (SRP)
Two key components involve of contranslational translocation:
1) signal-recognition particle (SRP)
- is a cytosolic ribonuclear protein particle - 300 nt RNA and 6 discrete (分開) polypeptides
- p54 bind to ER signal sequence in a nascent secretory protein - homologous to bacterial protein Ffh (hydrophobic residues) p54 - p9 and p14 interact with ribosome
- p68 and p72 are required for protein translocation
- SRP slows protein elongation when microsomes are absent 2) SRP receptor
- integral membrane protein (an α subunit & smaller a β subunit) - protease – releasing soluble form of the SRP receptor
- p54 of SRP and α subunit of receptor - GTP – promote interaction - GTP hydrolysis – fidelity (忠誠的)
Signal-recognition particle (SRP)
p9 and p14 interact with ribosome - p68 and p72 are required for protein translocation
hydrophobic Signal peptide about hydrophobic
sequence
Bind to signal peptide
Passage of growing polypeptide through the translocon is driven by energy released during translation
Mammalian translocon:
Sec61 complex Sec61α
- integral membrane protein, - 10 membrane-spanning α helixes
- interact with translocating peptide(chemical cross- linking exp.)
Sec61β, sec61γ Signal peptidase
Electron microscopy reconstruction reveals that a translocon associates closely with a ribosome
Sec61
ribosome
Energy needs during protein translocation
1. Unfolding the proteinin the original location
co-translation translocation: chain elongation during translation post-translational translocation/mitochondrial import:
chaperone (Hsp70) unfolds protein in an ATP-dependent manner
2. Opening of the “gate”
mutual stimulation of GTPaseactivities of an SRP subunit (p54) and the α subunit of SRP-receptor
3. Pulling through the channel:
chaperone activity inside the target organelle (Hsp70) that in addition helps fold the protein
ATP hydrolysis powers post-translational translocationof some secretory proteins in yeast
In most eukaryotes, secretory proteins enter ER by co-translational translocation, using energy form translation to pass through the membrane. But Yeast, post-translational translocation.
BiP is HSP 70 familyof molecular chaperones, a peptide- binding domain and an ATPase domain. For bind and stabilized unfolded or folded protein.
BiP
Molecular chaperones
Up-regulated during heat shock, conserved 2 classes
Hsp70: protect a misfolded or unfolded protein from degradation/folding, Hsp40 and Hsp90 as cofactors Hsp60 (chaperonin), actively helps protein folding Organelle specific, e.g. Bip in the ER
16.2 Insertion of proteins into the ER membrane
How integral proteins can interact with membranes?
Topogenic sequence, for basic mechanism used to translocated solube secretory proteins across the ER membrane
Most important: the hydorphobicsequence for interaction with intra-membrane
Single-pass Multipass
Moving Proteins into Membranes and Organelles (protein targeting)
Fig 16-11 Synthesis and insertion into the ER membrane of type 1 single-pass proteins
Type I: cleavable N-terminal signal sequence (SS), stop-transfer sequence in the C-terminal portion of the protein
most of the protein is on the exoplasmic side
similar to type III, except that there is no signal sequence Insertion into the ER membrane of type I proteins
Most cytosolic transmembrane proteins have an N-terminal signal sequence and an internal topogenic sequence
Type III also has
Fig 16-12 Synthesis and insertion into the ER membrane of type II single-pass proteins
Type II: no SS, stop-transfer sequence, start-transfer sequence in the N- terminal portion, often (+) charge N-terminal to the hydrophobic domain
A single internal signal-anchor sequence directs insertion of single-pass Type II transmembrane proteins
Positive charge amino acids face to cytosol ??
Synthesis of a single pass transmembrane protein with the C-terminal domain in the lumen
Type II: no SS, stop-transfer sequence, start-transfer sequence in the N- terminal portion, often (+) charge N-terminal to the hydrophobic domain
Insertion into the ER membrane of type III proteins
High density of positively charged aa at one end of the signal-anchor sequence determine insertion orientation.
Type III: no SS, stop-transfer sequence, flanked by +-charged residues on its C- terminal side
same orientation as type I, but, synthesized without SS, often (+) charge C-terminal to the hydrophobic domain
Synthesis of mutiple pass transmembrane protein Type IV: multipass membrane protein (various options)
Type IV: multipass membrane protein (various options)
Type I: cleavable N-terminal SS (signal sequence),
stop-transfer sequence in the C-terminal portion of the protein
most of the protein is on the exoplasmic side Type III: same orientation as type I, but, synthesized
without SS, often (+) charge C-terminal to the hydrophobic domain
Type II: no SS, start-transfer sequence in the N-terminal portion, often (+) charge N-terminal to the hydrophobic domain
Type IV: multipass membrane protein
GPI (glycosylphosphatidylinositol): Type I protein is cleaved and the lumenal portion is transferred to a
GPI (glycosylphosphatidylinositol): Type I protein is cleaved and the lumenal portion is transferred to a preformed lipid anchor.
After insertion into the ER membrane, some proteins are transferred to a GPI anchor
Arrangement of topogenic sequences in type I, II, III and IV proteins.
Even number of a helices: N- & C-termini on the same side; type IV-B: on the opposite sides.
Whether α helix functions as signal-anchor sequence or stop-transfer anchor sequence is determined by its order
+++ prefer cytosol (mechanism ?) Usually, enter cytosol
Usually, enter lumen Still move
Protein targeting to ER A phospholipid anchor tethers some cell surface proteins to
the membrane:GPI-anchored proteins GPI
(glycosylphosphatidylinositol)- anchored proteins can diffuse in lipid bilayer.
GPI targets proteins to apical membrane in some polarized epithelial cells.
The topology of a membrane protein often can be deduced from its sequence: hydropathy profile (親水性行為)
Hydropathic index for each aa.
Total hydrophobicity of 20 contiguous aa
hydrophobicity
Usually for cytosol
16.3 Protein modifications, folding, and quality control in the ER
1. Addition and processing of carbohydrates (glycosylation) in the ER and Golgi
2. Formation of disulfide bondsin the ER
3. Proper folding of polypeptide chains and assembly of multisubunitproteins in the ER
4. Specific proteolytic cleavagesin the ER, Golgi, and secretory vesicles
m-RNA → ribosome-ER → peptide → modification → mature protein
Fig 16-16 Common 14 residue precursor of N-linked oligosaccharide that is added to nascent proteins in the rough ER
A preformed N-linked
oligosaccharide is added to many proteins in the rough ER
Core region Glycosylation site: ERor golgicomplex All N-linkedoligosaccharides on secretor and membrane protein are conserved
N-linked: complex
O-linked: one to four sugar residues
Glycosylation (醣基化)
There are two basic types of glycosylation which occur on:
(a)N-linked: asparagine (b)O-linked: serineand
threonine
Biosynthesis of dolichol pyrophosphoryl oligosaccharide precursor
Strongly hydrophobic lipid (79-95 carbon)
Oligosaccharide side chain may promote folding and stability of glycoproteins
Consensus:
Asn-X-Ser/Thr (x: did not proline)
UDP-N-acetylglucosamine
The antibiotic tunicamycin acts by mimicking the structure of UDP- N-acetylglucosamine, the substrate in the first enzymatic step in the glycosylation pathway.
It thus blocks protein post-translational modification and hence protein production is inhibited to kill eukaryotic cells.
Addition & processing of N-linked oligosaccharides in r-ER of vertebrate cells
The molecule is flipped from the ER membrane to the ER
lumen.
Additional sugars are added via dolichol phosphate. Finally, the oligosaccharide (14 residues) is transferred to a specific Asn in the lumen.
Before the glycoprotein leaves the ER lumen three glucose units are removed (part of the folding process).
Cytoplasm Lumen
N-glycosylation: Oligosaccharide precursor is attached to the protein co-translationally
Red: GlcNAc Blue: mannose Green: Glucose Consensus:
Asn-X-Ser/Thr
Function of the ER: Glycosylation
Protein glycosylation serves several functions.
Promote proper folding: e.g. influenza virus hemagglutinin cannot fold properly in the presence of tunicamycin or a mutation of Asn to Gln.
Confer stability.
Involved in cell-cell adhesion; Cell adhesion molecules (CAMs).
Protein glycosylation takes place in the ERand Golgi The endoplasmic reticulum- ER
– A continuous cytoplasmic network studded with ribosomes and functions as a transport system for newly synthesized proteins.
The Golgi complex
– An organelle consisting of stacks of flat membranous vesicles that modify, store, and route products of the ER.
N-linked glycosylation begins in the ER and continues in the Golgi apparatus (via dolichol phosphate).
O-linked glycosylation takes place only in the Golgi apparatus.
In the Golgi:
1. O-linked sugar units are linked to proteins.
2. N-linked glycoproteins continue to be modified.
3. Proteins are sorted and are sent to- lysosomes
secretory granules plasma membrane
according to signals encoded by amino acid sequences.
Glycoproteins
Carbohydrates can be covalently linked to proteins to form glycoproteins.
– These proteins have a low percentage of carbohydrate when compared to proteoglycans.
Carbohydrates can be linked through the amide nitrogen of asparagine (N- linkage),
or through the oxygen of serine or threonine (O-linkage).
2 classes of glycosylation
O-linked: N-acetylgalactosamine linked to Ser/ThrGenerally short (1-4 sugars) Sugars added sequentially
N-linked:N-acetlyglucosamine linked to AsnComplex Preformed
oligosaccharide added in ER Modified by addition/removal of sugars in ER and Golgi
Disulfide bondsare formed and rearranged by proteins in the ER lumen via protein disulfide isomerase (PDI) -SH: sulfhydryl group
55 kDa protein -acts as dimer, contains protein binding site
Cystine
Formation of disulfide bonds in eukaryotes & bacteria
Transfers electrons to a disulfide bond in the luminal protein
PDI like DsbA
Functions of The ER
Chaperones: BiP Glycosylation GPI-linkages
Disulfide bond formation Proper Folding -Quality Control Multisubunit (multimeric) assembly Specific proteolytic cleavages Secretory vesicles
ER proteins that facilitate folding & assembly of proteins.
Chaperones and other protein facilitates folding and assembly of proteins
BiP: a chaperone that prevents nascent chain from misfolding or forming aggregates.
PDI: stabilizes proteins with disulfide bonds.
Calnexin & calreticulin: lectins that bind a single glucose attached onto unfolded or misfoldedpolypeptide chains and prevent their aggregation.
(p677)
Peptidyl-prolyl isomerase: facilitates folding by accelerating rotation about peptidyl-prolyl bonds.
In all cases, multimeric constituting in ER
Unfolded protein vs. ER quality control
整理
lectins
A glucosyl transferase can recognize an unfolded protein and add one terminal glucose to it
Carbohydrate binding protein
16-18 3a
An example: folding & assembly of hemagglutinin trimer in ER
Quality control by BiP & calnexin:
ensuring that misfolded proteins do not leave ER.
Hemagglutinin它是負責與sialic acid結合的蛋白,sialic acid本身是我們呼吸 道上體外面非常重要的抗原, ... 流行性感冒病毒必須先以Hemagglutinin與 呼吸道上體上的sialic acid結合,然後經過Engulf而進入到細胞內,當病毒 合成之後必須再把病毒釋放出去。
In addition to co-translational modifications, the correct folding/assembly may require the presence of a group of proteins called chaperones. Some chaperones (e.g. BiP) have high affinity toward unfolded proteins in general, yet others (e.g. calreticulin or calnexin) recognize more specific features (e.g. glycosylation) during the folding of a protein.
(Carbohydrate-binding proteins are called lectins. Calnexin and calreticulin are lectins binding to certain N-linked carbohydrates)
ER
Golgi Vesicles
Cell Surface
The orientation of a membrane protein is established during synthesis on the ER membrane
lumen
Extracellular space
What if unfolded proteins start to accumulate within a cell?
unfolding in the cytosol:
leading to an increase of cytosolic chaperones (also called heat shock response)
unfolding in the ER:
leading to an increase of ER chaperones (also called unfolding protein response, UPR)
• Translocated proteins can be exported to the cytosol.
• There they are:
– ubiquitinated
– degraded by the proteasome
—a process known as ER-associated degradation.
The unfolded-protein response: increased expression of protein-folding catalysts.
1. Unfolded protein ↑ → binding to Bip
2. Ire1 (left) no bind to Bip → dimerization → activate endonuclease activity 3. Endonuclease → spliced
immature Hac1 mRNA → Hac1 mRNA
4. Hac1 mRNA
→transcription factor
→enter nucleus→ protein folding catalysts (ER chaperone gene transcript) Hac1
a transcription factor promoting the transcription of ER chaperone genes low expression in the absence of UPR high expression when the UPR is induced
expression level determined by the splicing of its mRNA in the cytosol
a kinase and endoribonuclease (cutting RNA)
unfolding protein response
Terminally misfolded proteins in the ER are returned to the cytosol for degradation Degradation of misfolded or unassembled proteins.
They are transported through the translocon back into cytosol and degraded by ubiquitin- mediated proteolytic pathway.
have very compact structures consisting of two α-helices and two β-sheet structures. The C- terminus of Ubiquitin is extended and unstructured.
Misfolded protein for ubiquitin-dependent proteasome degrade Unassembled or misfolded proteins are blocked from moving to the Golgi complex
ERAD: ER-associated degradation
misfolded proteins remain bound to ER chaperones (e.g., BiP, calnexin)
Aberrant (不正常) proteins are finally targeted for degradation and extruded back to cytoplasmic compartment through translocon
N-glycanase in cytosol removes N-linked carbohydrate moieties (去除一半) proteins are ubiquitinated in
cytosol and degraded via proteasome complex – ubiquitin-conjugating
enzymes are localized on cytoplasmic face of ER – Ub-conjugating enzymes
interact with integral membrane Ub ligases – polyubiquitinated proteins
are degraded in proteasomes
Emphysema 廣泛性肺泡肺氣腫
Misfolding protein in ER
The α1-antitrypsin mutation (release from hepatocytes, macrophage) trypsin → degrade → elastin (ECM) → support down
Anti-trypsin inhibited trypsin
proteolytic cleavages
The life cycle of misfoldedprotein (unfold protein response)
Pathway of protein breakdown in mammalian cells Cytosolic protein
Abnormal protein Short-lived protein ER-associated protein Long-lived protein
Endocytosed proteins Membrane protein Extracellular protein
Ubiquitin proteasome pathway
Lysosomal pathway
Degradation of protein
1. Lysosome: primarily toward extracellular protein and aged or defective organelles of the cells.
2. Proteasomes: Ubiquitin dependent; for intracellular unfolding, aged protein.
1. control native cytosolic protein
2. misfolded in the course of their synthesis in the ER
16.4 Export of bacterial proteins (post-translational translocation)
cytosol
Inner membrane
Periplasmic space
Cytosolic SecA ATPase pushes bacterial polypeptides through translocons into the periplasmic space.
Bacterial translocon is very similar to
eurkaryotic Sec61 complex G (-)
Several mechanisms translocate bacterial proteins into the extracellular space
The secretion mechanisms are important for pathogenic bactera → secreted extracellular protein to colonize specific tissue or host.
Four general types of bacterial secretion systems:
Type I and II: proteins translocated across to inner membrane → into periplasmic space → fold and disulfide bond formation → folded protein translocated → from periplasmic space to outer membrane by complex of periplasmic proteins; It need energy
Type III and IV: one step, large protein complex translocated directly from the cytosol to the extracellular space.
Type III
secretion apparatus for injecting bacterial proteins into eukaryotic cells.Pathogenic bacteria inject protein into animal.
Type III secretion is similar in size and morphology to the bacterial flagellum.
Yersinia pestis 鼠疫桿菌 人畜共通傳染病,為一古老的世界性瘟疫
Pathogenic bacteria inject protein → host cell → by type III model
16.5 Sorting of proteins to mitochondria and chloroplast All organelle are have lipid bilayer.
Mitochondrial or chloroplast DNA and ribosome → synthesized protein → correct subcompartment
The mechanisms of Sorting of protein to mitochondria and chloroplast is similar to bacteria
Amphiphatic N-terminal signal sequence direct proteins to the mitochondrial matrix:
Matrix-targeting sequences:
1. Located N-terminus 2. 20-50 amino acids in length
3. Rich in hydrophobic amino acids, positively charged amino acids (Arg, Lys), and hydroxylated ones (Ser, Thr)
4. Lack negatively charged acidic residues (Asp, Glu) 5. Alpha-helical conformation (one-hydrophobic,
opposite side – charged amino acids: amphipathic) 6. Amphipathicity of matrix-targeting sequences is
critical to their function
Fig 16-25 The post-translational uptakeof precursor proteins into mitochondria can be assayed in a cell-free system Export to mitochondria, not co-
translational translocation
The post-translational uptake of precursor proteins into mitochondria can be assayed in cell free system
The structure of mitochondria Mitochondrial protein import requires outer membrane receptor
and translocon in both membranes
chaperones
1. Unfolded protein binding chaperones,
2. Precursor protein bind to an import receptor, which contact with inner membrane
3. Transferred into import pore
4. Translocation protein 5. To adjacent channel in the
inner membrane 6. Translocated protein
binding to matrix chaperones, remove targeting sequenceby martix protease, and release chaperones.
7. Folding to mature protein
To matrix
Tom 20/22 ( import receptor) and Tom 40 (general import pore) Tim 23/17 proteins
Contact sites – close proximity
Tim 44 (translocation channel)/ Hsc70( a matrix chaperone) The interaction -ATP hydrolysis by matrix Hsc70
chaperonin- facilitate folding (yeast Hsc60 defect – fail to fold)
Molecular chaperons: which bind and stabilize unfolded or partly folded proteins, thereby preventing these proteins from aggregating and being degraded
Chaperonins: which directly facilitate the folding of proteins
Fig16-27 Experiments with chimeric proteins show that a matrix-targeting sequence alone directs proteins to the mitochodrial matrix and that only unfolded
proteins are translocated across both membranes Studies with chimeric proteins demonstrate important features
of mitochondial import: only unfolded protein can entery
DHFR(dihydrof olate reductase)
in the presence of chaperone
Matrix targeting sequence
No function sequecne
DHFR
Must unfold protein can enter mitochondrial matrix
MTX – binds tightly to the active site of DHFR and greatly stabilizes its folded conformation
Spacer sequence: >50 amino acids long Translocation intermediate is formed
<35 residues– intermediate
translocated proteins span both membranes in unfolded state
• Chemically cross-linking exp.
• 1000 general import pore (yeast mitochodria)
Can not entery
Precursor protein must be unfolded in order to traverse the import ores in the mitochondrial membranes
Matrix-targeting sequence alone directs proteins to mitochondrial matrix.
Only unfolded proteins are translocated across both membranes.
Bound to the translocation intermediate at a contact site
Three energy inputs are needed to import proteins into mitochondria
1. Cytosolic Hsc70-ATP hydrolysis - unfolding function 2. Matrix Hsc70-ATP hydrolysis – molecular motor to pull the
protein into the matrix (cf. chaperone BiP and Sec63 complex – in post-tranlational translocation into the ER lumen)
3. H+ electrochemical gradient (proton-motive force) across the inner membrane ( inhibitor or uncouple of oxidative phosphorylation such as cyanide or dinitrophenol, dissipates this proton motive force - proteins bind to receptor, but not be imported)
One hypothesis: positive charges in the amphipathic matrix-targeting sequences – electrophoresed or pulled into the matrix by inside- negative membrane electrical potential
Translocation into chloroplast occurs via a similar strategy to the one used by mitochondira
Both occur post-translationally
Both use two translocation complexes, one at each membrane Both require energy
Both remove the signal sequence after transfer
However chloroplasts have a H+ gradient across the thylakoid membrane and use GTP hydrolysis to drive transfer
Multiple signals and pathways target proteins to
submitochondrial compartments
Target:
1.inner-membrane 2.Intermembrane- space 3.Out-membrane: unknow mechanism
4.matrix
Matrix targeting
Outer-membrane proteins
•Short matrix-targeting sequence is followed by long stretch of hydrophobic amino acids
Inner-membrane proteins: three separate pathways (A)
Inner membrane has three pathway A,B,C
Inner-membrane proteins: three separate pathways (B) Inner-membrane proteins: three separate pathways (C)
No N-terminal matrix-targeting sequences
Three pathways for targeting inner-membrane proteins
Oxa1 also participates in the inner-membrane insertion of certain proteins encoded by mitochodrial DNA synthesized in matrix by mitochondrial ribosomes