Cytoskeleton-MF Flashcards
Smallest cytoskeletal filament
Microfilament
Actin filament
F-actin
MF are known for
Their role in muscle contraction
Building blocks of MF is
Actin
Actin is
Highly conserved globular protein
Actin types
Alpha-actin: muscle specific actin
Betta-actin: apical
Gamma-actin: basal
Most abundant in all eukaryotic cells
Actin
Actin after synthesis
Folds into globular U-shaped molecule that can bind ATP or ADP
G-actin (globular actin) will
Polymerize to form microfilaments (F-actin) filamentous actin
MF hydrolyze —— but MT hydrolyze ——
ATP
GTP
G actin can polymerize reversibly to
Filaments with lag phase, elongation phase
F-actin
Composed of two linear strand of polymerized G-actin wound into helix
All actin filament have——
Same orientation
Actin filament
-two stranded helical polymer
-concentrated beneath the plasma membrane
-easily change cell morphology by assembling/disassembling (Dynamic)
-MF have polarity
-Requires ATP (actin is ATPase)
Polarity of MF reflect on
More rapid addition of G-actin at + end than - end
After G actin assembles onto MF,
ATP bound to them slowly hydrolyzed
hydrolysis is not required for polymerization
Growing end has
ATP actin
Most of Mf has
ADP actin
Treadmilling can occur in
MT and MF
Cells can regulate how MF are assembled such as
1- Cell that crawl: Lamellipodia and Filopodia at leading edge
2- cell that adhere tightly: stress fibers
—— organize and behave the MFs
Actin binding proteins
Actin binding proteins
1- ARP complex
2- Thymosin Betta 4
3- CapZ/ Tropomoduli
4- Formin
5- Cofilin
6- Filamin/ alpha-actinin
7- Gelsolin
8- Spectrin/Ankyrin
Thymosin Betta-4
Bind to (+ end) ATP-actin monomer and prevent them from polymerization
Profilin
Binds to ADP-G-actin catalyze the exchange ADP to ATP, promoting polymerization
Cofilin
Bind (- end) ADP-actin in MF, severe it promote depolarization
Capping proteins
CapZ: bind to + end, prevent addition/loss of subunit
Tropomodulin: bind to - end, prevent addition/loss of subunit
physically binding
Sever actin filaments
Gelsolin: break actin MF and caps the newly exposed + end
Cofilin: filament-severing proteins, facilitating depolarization
Formins
Control the assembly of actin filament
Nucleate polymerization of unbranched actin filament
Associated with + end, enabling rapid insertion of actin filament and protect the end from capping proteins
Some Formins have extensions that can bind
Profilin
Cross link filaments
Filamin: join two MF together
Bundle actin filament
Fimbrin: bundle Mf in microvilli
Link MF to membrane
One or more peripheral membrane proteins: spectrin and Ankyrin
Nucleate new filaments
Arp2/3 complex: nucleate new branches, tree-like network
Intermediate filament
Are not polarized
Support the cytoskeleton by acting as scaffold(bridging molecules) between Mt and MF
Intermediate filament assemble from
Fibrous Subunits rather than globular
Intermediate filament
-Central rod-shaped alpha-helix domain that is conserved
-Has N-terminus and C-terminus that differ greatly
-unlike MT and MF can be made of many different proteins
-have only been found in animals
- expression vary from tissue-to-tissue
Intermediate filament classified based on
Degree of AA
N terminus and c terminus sequence and structure can vary the most as they can binding site for
-intermediate filament
-MF
-MT
—— can help in diagnosis of certain tumors
Intermediate filament
Tumor cells
Lose morphological appearance but retain cytoskeletal proteins
By using of antibodies against IF can determine if origin is epithelial, mesenchymal, neural
IF proteins synthesize
-Primarily fibrous molecules, globular domain on each end
-two polypeptide spontaneously interact, wrapping around each other, create dimer
Two dimer assemble creating anti-parallel tetramer(no polarity)
Eight tetramers pack laterally to form one unit
Unit associate with each other to form elongated IF
Unlike MF and MT, in IF,
Units are not removed from ends, rather the interior
IF assembly does not
Require energy (ATP or GTP)
IF mechanical strength
MT resist bending
MF generate tension
IF elastic and withstand tensile forces
IF are most stable cytoskeleton component
IF are not static, dynamically remodeled
IF examples
Nuclear Matrix
Nuclear Lamina
Both supporting structures
Nuclear matrix (nucleoskeleton)
Insoluble fibrous network
Maintains the shape of nucleus
Attachment site for chromatin
Nuclear Lamina
Thin dense fibers lining the inner surface of inner nuclear membrane
-made of IF composed of Lamina
-disassemble prior to mitosis by phosphorylation of Lamins
