Transcript Document

Chapters 6
The Cytoplasm & Organelles
The Typical Cell
• typical cell: 1. nucleus
2. cell membrane
3. cytoplasm
-cytosol
-cytoskeleton
4. cytoplasmic organelles
-membranous
-non-membranous
Cytoplasm
•
•
semi-fluid-like jelly within the cell
division into three subdivisions: cytosol, cytoskeleton &
organelles
The Cytosol – Eukaryotic Cells
• eukaryotic cells – part of the cytoplasm
– about 55% of the cell’s volume
– about 70-90% water PLUS
•
•
•
•
•
ions
dissolved nutrients – e.g. glucose
soluble and insoluble proteins
waste products
macromolecules and their components - amino acids, fatty
acids
• ATP
• unique composition with respect to extracellular
fluids
Cytosol
•higher K+
•lower Na+
•higher concentration
of dissolved and
suspended proteins
(enzymes, organelles)
•lower concentration
of carbohydrates
(due to catabolism)
•larger reserves of amino
acids (anabolism)
ECF
•lower K+
•higher Na+
•lower concentration
of dissolved and
suspended proteins
•higher concentration
of carbohydrates
•smaller reserves of amino
acids
Cytoskeleton:
•internal framework of the cell
•gives the cytoplasm flexibility and strength
•provides the cell with mechanical support
•gives the cell its shape
•can be rapidly disassembled in one area of
the cell and reassembled in another
•anchorage points for organelles and cytoplasmic
enzymes
•also plays a role in cell migration and movement
by the cell
The Cytoskeleton and Cell motility
• motility = changes in cell location and the limited movements in
parts of the cell
• the cytoskeleton is involved in many types of motility
• requires the interaction of the cytoskeleton with motor proteins
Vesicle
• some roles of motor proteins:
ATP
• 1. motor proteins interact with
microtubules (or microfilaments) and vesicles
to “walk” the vesicle along the cytoskeleton
(a)
• 2. motor protein, the cytoskeleton and
Microtubule
the plasma membrane interact to
move the entire cell along the ECM
• 3. motor proteins result in the
bending of cilia and flagella
(b)
Receptor for
motor protein
Motor protein
(ATP powered)
Vesicles
Microtubule
of cytoskeleton
0.25 m
Cytoskeleton:
•three major components
1. microfilaments
2. intermediate filaments
3. microtubules
10 m
10 m
5 m
Column of tubulin dimers
Keratin proteins
Fibrous subunit (keratins
coiled together)
Actin subunit
25 nm
7 nm


Tubulin dimer
812 nm
1. microfilaments = thin filaments made up of a protein called actin
-twisted double chain of actin subunits
-forms a dense network immediately under the PM (called the cortex)
-also found scattered throughout the cytoplasm
1. microfilaments =
-function: 1. anchor integral proteins and attaches them to the cytoplasm
2. interaction with myosin = interacts with larger microfilaments made up of myosin
- results in active movements within a cell (e.g. muscle cell contraction)
3. provide much of the mechanical strength of the cell – resists pulling forces within
the cell
4. give the cell its shape
5. also provide support for cellular extensions called microvilli (small intestines)
Examples of Actin/Myosin:
Muscle cell
0.5 m
Actin
filament
In muscle cells – motors within filaments
made of myosin “slide” along filaments
containing actin = Muscle Contraction
Myosin
filament
Myosin
head
(a) Myosin motors in muscle cell contraction
Cortex (outer cytoplasm):
gel with actin network
100 m
Inner cytoplasm: sol
with actin subunits
Extending
pseudopodium
In amoeba – interaction of actin with myosin
causes cellular contraction and pulls the
cell’s trailing edge (left) forward
-can also result in the production of
Pseudopodia (for locomotion, feeding)
(b) Amoeboid movement
Chloroplast
(c) Cytoplasmic streaming in plant cells
In plant cells – a layer of cytoplasm cycles
around the cell
-streaming over a “carpet” of actin filaments
may be the result of myosin motors attached
to organelles
30 m
2. intermediate filaments = more permanent part of the cytoskeleton than other
filaments
-five types of IF filaments – type I to type V
-made up of proteins such as vimentin, desmin, or keratin
-each cell type has a unique complement of IFs in their cytoskeleton
- all cells have lamin IFs – but these are found in the nucleus
-some cells also have specific IFs
-
e.g neurons also posses IFs made of neurofilaments
type I IFs = acidic keratins
type II IFs = basic keratins
type III IFs = desmin, vimentin
type IV IFs = neurofilaments
type V IFs = nuclear lamins
kidney cell - vimentin
2. intermediate filaments =
function: 1. impart mechanical strength to the cytoskeleton – specialized for bearing tension
(like microfilaments)
2. support cell shape
e.g. forms the axons of neurons
3. anchor & stabilize organelles
e.g. anchors the nucleus in place
4. transport materials
e.g. movement of neurotrasmitters
into the axon terminals
3. microtubules = hollow rods or “straws”
- made of repeating units of proteins called tubulin
- function: 1. cell shape & strength
2. organelles: anchorage & movement
3. mitosis - form the spindle (chromosome movement)
4. form many of the non-membranous organelles
- cilia, flagella, centrioles
-tubulin
-tubulin
components of:
1. mitotic spindle
2. cilia and flagella
3. axons of neurons
3. microtubules -the basic microtubule is a hollow cylinder = 13 rows of tubulin called
protofilaments
-tubulin is a dimer – two slightly different protein subunits
- called alpha and beta-tubulin
-alternate down the protofilament row
-tubulin
-tubulin
-animal cells – microtubule assembly occurs in the MTOC (microtubule organizing
center or centrosome)
-area of protein located near the nucleus
-within the MTOC/centrosome :
1. a pair of modified MTs called centrioles
2. pericentriolar material – made up of factors that mediate microtubule
assembly
3. “-” end of assembling microtubules (MTs grow out from the centrosome)
-other eukaryotes – there is no MTOC
-have other centers for MT assembly
•can be found as a single tube
a doublet and a triplet
Microtubule Assembly within the MTOC:
-MTs are easy to assemble and disassemble – by adding or removing tubulin dimers
-one end accumulates or releases tubulin dimers much faster than the other end called the
plus end
-the tubulin subunits bind and hydrolyze GTP – determines how they polymerize into the MT
-MT disassembly is a mechanism of
certain chemotherapy drugs
http://www.nature.com/nrc/journal/v4/n4/fig_tab/nrc1317_F4.html
Non-membranous Organelles
A. Centrioles: short cylinders of tubulin
- 9 microtubule triplets
-called a 9+0 array (9 peripheral triplets, 0 in the center)
-grouped together as pairs – arranged perpendicular to one another
-make up part of the centrosome or MTOC
-role in MT assembly??
-also have a role in mitosis - spindle and chromosome alignment
B. Cilia & Flagella
• cilia = projections off of the plasma membrane of eukaryotic cells – covered with PM BUT
NOT MEMBRANOUS ORGANELLES
• beat rhythmically to transport material – power & recovery strokes
• found in linings of several major organs covered with mucus where they function in
cleaning
e.g. trachea, lungs
Trachea
B. Cilia & Flagella
• cytoskeletal framework of a cilia or flagella = axoneme (built of microtubules)
• contain 9 groups of microtubule doublets surrounding a central pair= called a 9+2 array
• cilia is anchored to a basal body just beneath the cell surface
0.1 m
Outer microtubule
doublet
Dynein proteins
Central
microtubule
Radial
spoke
Microtubules
Plasma
membrane
Cross-linking
proteins between
outer doublets
(b) Cross section of
motile cilium
Basal body
0.5 m
(a) Longitudinal section
of motile cilium
0.1 m
Triplet
(c) Cross section of
basal body
Plasma membrane
•flagella = resemble cilia but much larger
• 9+2 array
• found singly per cell
• functions to move a cell through the ECF
-DO NOT HAVE THE SAME STRUCTURE AS BACTERIAL FLAGELLA
Cilia, Flagella and Dynein “motors”
• in flagella and motile cilia – flexible cross-linked
proteins are found evenly spaced along the
length
– blue in the figure
• these proteins connect the outer doublets to
each other and to the two central MTs of a 9+2
array
• each outer doublet also has pairs of proteins
along its length
– these stick out and reach toward its neighboring
doublet
– called dynein motors
– responsible for the bending of the microtubules of
cilia and flagella when they beat
Microtubule
doublets
Cross-linking proteins
between outer doublets
Dynein protein
Cilia, Flagella and Dynein “motors”
•
dynein “walking” moves flagella and cilia
− dynein protein has two “feet” that walk along the MT
− dyneins alternately grab, move, and release the outer
microtubules
− BUT: without any cross-linking between adjacent MTs
- one doublet would slide along the other
− elongate the cilia or flagella rather than bend it
− so to bend the MT  must have proteins crosslinking between the MT doublets (blue lines in
figure)
– protein cross-links limit sliding
– forces exerted by dynein walking causes doublets to
curve = bending the cilium or flagellum
Microtubule
doublets
ATP
Dynein protein
(a) Effect of unrestrained dynein movement
Cross-linking proteins
between outer doublets
ATP
Anchorage
in cell
(b) Effect of cross-linking proteins
1
3
2
(c) Wavelike motion
Membranous Organelles
• completely surrounded by a phospholipid bilayer similar to the
PM surrounding the cell
• allows for isolation of each individual organelle - so that the
interior of each organelle does not mix with the cytosol
-known as compartmentalization
• BUT - cellular compartments must “talk” to each other
• therefore the cell requires a well-coordinated transport system in
order for the organelles to communicate and function together
-”vesicular transport”
-active process – requires ATP
Membranous Organelles
•major functions of the organelles
•1. protein synthesis – ER and Golgi
•2. energy production – mitochondria
•3. waste management – lysosomes and peroxisomes
Membranous Organelles
•
•
•
•
the organelles of a eukaryotic cell are not constructed de novo
they require information in the organelle itself
when a cell divides – it must duplicate its organelles also
in general – the cell enlargens existing organelles by incorporating new
phospholipids and proteins into them
• the bigger organelle then divides when the daughter cell divides during
cytokinesis
The Endomembrane System: A Review
• endomembrane system is a
complex and dynamic player in
the cell’s compartmental
organization
• divides the cell into
compartments
• includes the:
–
–
–
–
–
Nucleus
Endoplasmic Reticulum
Golgi apparatus
lysosomes, endosomes
vacuoles and vesicles
The Endomembrane System: A Review
• proteins travelling
through the ER and Golgi
are destined for
– 1. Secretion outside the
cell
– 2. Plasma membrane
– 3. Lysosome
1. Endoplasmic reticulum (ER) = series of membrane-bound,
flattened sacs in communication with the nucleus and the PM
-each sac or layer = cisternae
-inside or each sac = lumen (10% of total cell volume)
-distinct regions of the ER are functionally specialized – Rough ER
vs. Smooth ER
-three functions:
1. synthesis – phospholipids, lipids and proteins
• proteins
• phospholipids & lipids
• 2. storage – intracellular calcium
• 3. transport – site of transport vesicle production
1. Endoplasmic reticulum (ER)
-two types: Rough ER - outside studded with ribosomes
-continuous with the nuclear membrane
-protein synthesis, phospholipid synthesis
-also the initial site of processing and sorting of proteins
1. Endoplasmic reticulum (ER)
-the import of proteins into the RER is a co-translational process
-import of proteins into an organelle = translocation
-proteins are imported as they are being translated by
ribosomes
-in contrast to the import of proteins into other organelles
(e.g. chloroplasts, mitochondria, peroxisomes) and the
nucleus = post-translational process
Co-translational Protein Synthesis
two kinds of proteins enter the ER:
1.ER proteins – transmembrane proteins that stay
stuck in the ER membrane PLUS ER lumen proteins
that remain in the ER
2. proteins destined for the Golgi, PM or lysosome
or secretion
Co-translational
Protein Synthesis
• transport from the ribosome across the ER membrane
requires the presence of an ER signal sequence (red in
the figure)
• 16-30 amino acids at the beginning of the peptide
sequence (N-terminal)
Co-translational
Protein Synthesis
• a complex of proteins will bind this signal in the cytoplasm = signal recognition
particle/SRP
• the ER membrane has receptor for the SRP and ribosome – SRP receptor (yellow
protein in figure)
• the ribosome is “docked” next to a “hole” in the ER membrane (blue protein in
figure) = translocon
• translocon recognizes the signal sequence and binds it  “guides” the rest of the
translating polypeptide into the ER lumen
• once the polypeptide is fed into the ER lumen – a peptidase (located in the SRP
receptor complex) cleaves the signal sequence off
Translocation
• try this animation – it might be a bit complicated – but give it a
try anyway
• http://www.rockefeller.edu/pubinfo/proteintarget.html
• here’s a figure from a molecular biology text that summarizes
the process
• once the polypeptide is fed into the ER lumen – a peptidase cleaves
the signal sequence off = PRODUCES A SOLUBLE PROTEIN
– localizes to the ER lumen
• the presence of another sequence of amino acids within the
polypeptide – stop-transfer sequence – the translocator stops
translocating and transfers the polypeptide into the ER membrane =
PRODUCES A TRANSMEMBRANE PROTEIN
Modifications in the RER
• 1. folding of the peptide chain
– actually a spontaneous process – due to
the side chains on the amino acids
– only properly folded proteins get
transported to the Golgi for additional
processing and transport
– many proteins located in the ER which
supervise this folding
• 2. formation of disulfide bonds
– help stabilize the tertiary and
quaternary structure of proteins
Modifications in the RER
• 3. breaking of specific
peptide bonds – proteolytic
cleavage or proteolysis
• 4. assembly into multimeric
proteins (more than one
chain)
for an animation go to
http://sumanasinc.com/webcontent/animatio
ns/content/proteinsecretion_mb.html
Modifications in the RER
• 5. addition and processing of carbohydrates = glycosylation
– N-linked glycosylation = attachment of 14 sugar residues as a group to an
asparagine amino acid within the protein
– the sugar is actually built and then transferred as one unit to the nearby
translating protein by a transferase protein
– needs to be trimmed down in order to allow protein folding
most proteins made
in the ER undergo
N-linked glycosylation
1. Endoplasmic reticulum (ER)
Smooth ER – extends from the
RER
-free of ribosomes
main function is transport
vesicle synthesis – area where
this happens can be called
transitional ER
-but other cell types have SER with enzymes
embedded in it for additional functions:
1. lipid and steroid biosynthesis for
membranes
2. detoxification of toxins and drugs
3. cleaves glucose so it can be released into
the bloodstream
4. uptake and storage of calcium
2. Ribosomes = can be considered a nonmembranous organelle
• made in the nucleolus
•2 protein subunits in combination with rRNA
-large subunit = 28S rRNA, 5.8S rRNA, 5 rRNA + 50 proteins
-small subunit = 18S rRNA + 33 proteins
•proteins are translating in the cytoplasm and imported into the nucleus
•rRNA is transcribed in the nucleolus
•ribosomes found in association with the ER = where the peptide strand is fed into
from the ribosome
•also float freely within the cytoplasm as groups = polyribosomes
3. Golgi Apparatus = stacks of membranes called cisternae (cisterna, singular)
-the first sac in the stack = cis-face (faces the ER)
-the last sac in the stack = trans-face
-the ones in the middle = medial cisterna or cisternae
Named after Camillo
Golgi in 1897
3. Golgi Apparatus
-associated with the cis and trans faces are additional networks of interconnected
cisternal structures
-called the cis Golgi network (CGN) and trans Golgi network (TGN)
-the TGN has a critical role in protein sorting
3. Golgi Apparatus
•site of final protein modification and packaging of the finished protein
•functions:
•1. protein modification
•A. glycosylation - creation of glycoproteins and proteoglycans
•B. site for phosphate addition to proteins = phosphorylation
•C. protein trimming
•2. production of sugars
•Golgi makes many kinds of polysaccharides
•3. formation of the lysosome
•4. packaging of proteins and transport to their final destination
•TGN acts as a sorting station for transport vesicles
Modifications in the Golgi
• glycosylation = produces a glycoprotein or
a proteoglycan
– most plasma membrane and secreted proteins
have one or more carbohydrate chains
– sugars help target proteins to their correct
location; are important in cell-cell and cellmatrix interactions
– two kinds: N-linked and O linked
– O-linked sugars are added one at a time in the
Golgi to the amino acids serine, threonine or
lysine (usually one to four saccharide subunits
total)
– N-linked sugars are added as a group (about 14
sugars!) in the ER
Proteoglycan
• glycosylation:
– glycosylation starts in the ER
• N-linked glycosylation – addition of Nlinked oligosaccharides
• many of these N-linked sugar residues
are trimmed off within the ER
• important for folding of the protein
– glycosylation continues in the
cisternae of the Golgi
• addition of O-linked oligosaccharides
to proteins
• PLUS modification of the N-linked
oligosaccharides - either addition or
removal of sugar residues
Why Glycosylation?
• the vast abundance of glycoproteins suggests that glycosylation has an
important function
• N-linked is found in all eukaryotes – including single-celled yeasts
• a type of N-linked can even be found in archaea – in their cell walls
• WHY GLYCOSYLATION?
• N-linked in the ER is important for proper protein folding
• N-linked also limits the flexibility of the protein
• the sugar residues can prevent the binding of pathogens
• sugar residues also function as signaling chemicals
• sugar residues function in cell interactions
Why Glycosylation?
• O-linked glycosylation
– O-linked are added one at a time in the Golgi to the amino
acids serine, threonine or lysine (one to four saccharide
subunits total)
• added on by enzymes called glycosyltransferases
• human A, B and O antigens are sugars added onto proteins and lipids in
the plasma membrane of the RBC
• everyone has the glycosyltransferase needed to produce the O antigen
• those with blood type A have an additional Golgi glycosyltransferase
enzyme which modifies the O antigen to make the A antigen
• a different glycosyltransferase is required to make the B antigen
• both glycosyltransferases are required for the creation of the AB
antigen
• coded for by specific gene alleles on chromosome 9 (ABO locus)
Modifications in the Golgi: Protein Trimming
some PM proteins and most secretory proteins are synthesized
as larger, inactive pro-proteins that will require additional
processing to become active
this processing occurs very late in maturation
◦
◦
◦
◦
processing is catalyzed by protein-specific enzymes called proteases
some proteases are unique to the specific secretory protein
trimming occurs in secretory vesicles that bud from the trans-Golgi face
processing could be at one site (albumin) – other proteins may require
more than one peptide bond (insulin)
The Golgi: Protein transport within the
cytoplasm
protein transport within the cell is tightly regulated
most proteins usually contain “tag” or signals that tell them where
to go
in the Golgi - specific sequences within a protein will cause:
1. retention in the Golgi
2. will target it to lysosomes
3. send it to the PM for fusion
4. send it to the PM for secretion
a lack of a signal means you will automatically be secreted =
constitutive secretion
SO - WHERE DO PROTEINS GO AFTER THE
GOLGI???
-proteins budding off the Golgi have three targets:
targets:
1. secretory vesicles for
exocytosis
2. membrane vesicles for
incorporation into PM
3. transport vesicles for the
lysosome
WHAT IF YOU AREN’T ONE OF THESE PROTEINS??
ER proteins stay in the ER
◦ never traffic to the Golgi
◦ these ER proteins will have a retention signal
Ribosomal proteins
◦
◦
◦
◦
◦
translation of ribosomal proteins are done in the cytoplasm by polyribosomes
assembled into the large and small protein subunits in the cytoplasm
imported into the nucleus
rRNAs are transcribed in the nucleolus - no translation!!!
protein subunits and rRNAs are assembled in the nucleolus to form the small and
large ribosomal subunits – exported from nucleus
mitochondrial proteins
◦ the mitochondria has its own DNA, transcribes its own mRNA and has its own
ribosomes for translation
4. Lysosomes = “garbage disposals”
-dismantle debris, eat foreign invaders/viruses taken in by
endocytosis or phagocytosis
-also destroy worn cellular parts from the cell itself and recycles
the usable components = autophagy
-form by budding off the trans-Golgi network??
-cell biologists not really sure exactly how the lysosome forms
4. Lysosomes
-contains powerful enzymes to breakdown substances into their component parts
-over 40 kinds of hydrolytic enzymes
-these enzymes are collectively known as acid hydrolases
-acidic interior - critical for function of these enzymes
-the hydrolytic enzymes of the lysosome need to be cleaved
first in order to become enzymatically active
-done by the acidity of the lysosomes interior
- acidic interior created and maintained by a hydrogen pump (H+ ATPase)
that pumps H+ into the interior - Active transport
-chloride ions that diffuse in passively through a chloride channel - forms
hydrochloric acid (HCl)
4. Lysosomes
• several different kinds of lysosomes – diverse in shape and size
• types:
• lysosome – form from the budding and fusion of vesicles from the TGN
• these vesicles contain lysosomal enzymes
• early endosome – forms through receptor-mediated endocytosis from the
plasma membrane
• late endosome – forms by fusion of early endosomes with vesicles
containing lysosomal enzymes
• endolysosome – fusion of a late endosome with a pre-existing lysosome
• transforms it into a lysosome
• an endolysosome may be considered an immature lysosome
Diseases at the Organelle Level
Tay Sachs and lysosomes: also known a Hexosaminidase A deficiency
-named after Waren Tay and Bernard Sachs
-key identifying mark = cherry red spot in the retina
-lack one of the 40 lysosomal enzymes – hexosaminidase
-results in the accumulation of gangliosides (phospholipid) in the cell membrane
of neurons
-death of the neuron results  failure of nervous system communication
-infantile form of the disease = death by 4 yrs
-juvenile form = death from 5 to 15 yrs
-adult onset – not fatal; progressive loss of
nervous function
-most common in Ashkenazi Jews, French
Canadians and Cajun populations in
Lousiana (same mutation as Jews)
5. Mitochondria
-surrounded by a dual phospholipid
bilayer
• an outer mitochondrial membrane
• an inner mitochondrial membrane
• a fluid-filled space = mitochondrial
matrix (contains ribosomes!)
-the inner membrane is folded into
folds called cristae
-these increase the membrane surface
area for the enzymes of Oxidative
Phosphorylation
•outer membrane - 50% phospholipid & 50% protein
-very permeable - contains pores for the import and export of critical
materials
•inner membrane - 20% phospholipid & 80% protein
-less permeable vs. the outer membrane
-folded extensively to form partitions = cristae
-contains proteins that work to create an electrochemical gradient
-contains enzymes that use this gradient for the synthesis of ATP
-also contains pumps to move ATP into the cytosol
•matrix - lumen of the mitochondria
-breakdown of glucose into water and CO2
ends here (enzymes of the Transition
phase Kreb’s Cycle)
Cellular Respiration
-glycolysis
-transition phase
-citric acid cycle
-electron transport chain
http://biology.about.com/gi/dynamic/offsite.htm?site=http://www.sp.uconn.edu/%7Eterry/images/anim/ETS.html
http://biology.about.com/gi/dynamic/offsite.htm?site=http://www.biocarta.com/pathfiles/krebPathway.asp
http://vcell.ndsu.nodak.edu/animations/etc/movie.htm
6. Peroxisomes: found in all cells but abundant in liver and kidney
cells
-only identified in 1954
-may arise from pre-existing peroxisomes or may bud from the ER
-major function is oxidation (breakdown) of long chain fatty acids (beta-oxidation)
-results in the conversion of the fatty acid into acetyl coA  Kreb’s cycle
-in plant cells – beta-oxidation is only done by the peroxisome
-in animal cells – the mitochondria can also perform this reaction
-oxidation is done by oxidases = enzymes that use oxygen to oxidize substances
-remove hydrogen atoms from the fatty acid
-this reaction generates hydrogen peroxide (H2O2)
6. Peroxisomes: found in all cells but abundant in liver and kidney cells
PROBLEM #1: H2O2 is very corrosive
-therefore peroxisomes also contain an enzyme called catalase to break this
peroxide down into water and oxygen
PROBLEM #2: the electron transport chain in mitochondria produces superoxide
radicals (O2-) as a normal consequence of electron “leaking” (from complex I)
-peroxisomes also contain anti-oxidant enzymes to break down other
dangerous oxidative chemicals made by the cell during metabolism
e.g. SOD – breaks down O2- to make H2O2
-other functions of peroxisomes:
1. synthesis of bile acids
2. breakdown of alcohol by liver cells
Adrenoleukodystrophy and peroxisomes:
-X linked disorder in the gene ABCD1 (transporter protein)
-1:20,000 to 1:50,000 births
-in ALD - peroxisomes lack an essential enzyme
-leads to a build up of a long-chain saturated fatty acids on cells of throughout the body
-can results in the loss of the myelin sheath – not known why
-lethargy, skin darkens, blood sugar drops, altered heart rhythm imbalanced electrolytes,
paralysis, death
*** slowed by a certain triglyceride found in rapeseed oil
Lorenzo Odone = “Lorenzo’s Oil” (mixture of unsaturated fatty acids
that slows the development
of these saturated FAs)
F-actin and peroxisomes