exam 3b Lecture 28 Flashcards
How is skeletal muscle cell specialized?
Myoblasts fuse to produce syncytia and bundles of aligned myofibrils
Syncitium
Multinucleated cell
What makes up skeletal muscle cell?
Sarcolemma, mitochondria and myofibrils
What is contractile unit in myofibril?
Sarcomere
Structure of sarcomere
Z discs on outside, positive on outside ends; light bands on outside, dark on inside, M line in middle. Thick filament is myosin and thin filament is actin.
How is myosin held?
By connection to large protein called titin.
Length of sarcomere
2.2 micrometers
what end of actin filament attaches to z disc of sarcomere?
Positive end
When end of actin filament is towards middle?
Minus end
What caps the actin filament at the z disc?
Cap Z
To which protein is myosin (thick filament) attached?
Titin
What is thin filament?
Actin
Where does myosin switch polarity?
At M line
Nebulin
Enormous protein that influences precise length of each thin filament. Consists almost entirely of a repeating 35 aa actin-binding motif. Stretches from the z disk toward the minus end of each thin filament, which is capped and stabilized by tropomodulin.
What coats actin filament besides nebulin?
Tropomyosin and troponin
Tropomodulin
Caps the minus end of the actin filaments and capZ anchors plus end at the Z disc, which also contains alpha-actinin.
In sliding filament model of muscle contraction, what is contracting? Towards what is there movement?
Sarcomere is contracting, not filaments. Movements towards plus ends at z disc.
What two components regulate myosin cycle in a muscle cell?
Sarcoplasmic reticulum and t-tubule
Sarcoplasmic reticulum
Specialized ER that surrounds each myofibril
T-tubule
Invagination of plasma membrane. Makes close contact with SR.
What kind of channel is on T-tubule membrane? How is it gated?
Dihydropyridine receptor (L-type Ca2+ channel). It is voltage gated.
What makes up lumen of T-tubule?
Extracellular space
What is between the T-tubule and the SR membrane?
Cytosol
What receptor is on SR membrane?
Ryanodine receptor
What happens to ryanodine receptor when L-Channel is opened and activated by calcium?
The conformation of the ryanodine receptor changes so that calcium ion can escape SR and go into cytosol.
What drives the opening of the L-channel and the release of calcium ion from SR?
Action potential
How long is distance between T-tubule membrane and SR membrane?
35nm
how does ryanodine receptor activate under positive feedback?
Released Ca2+ feeds back to ryanodine receptor to release more calcium ion.
How are skeletal and heart muscle different when it comes to ryanodine receptor activity?
Skeletal muscle has physical interaction of l chanel with ryanodine receptor cause calcium ion release. In heart muscle, calcium ion influx through dihydropyridine receptor activates the ryanodine receptor to release Ca2+
Where are troponin complex and tropomyosin located on actin filament?
Along the actin filament.
What makes up troponin complex?
polypeptides ICT. Troponin I binds to actin as well as to troponin T. In a resting muscle, the troponin I-T complex pulls the tropomyosin out of its normal binding groove into a position along the actin filament that interferes with the binding of myosin heads, thereby preventing any force-generating interaction. When Ca2+ level is raised, troponin C, which binds up to four molecules of Ca2+, causes troponin I to release its hold on actin. This allows tropomyosin molecules to slip back into their normal posotion so that the myosin heads can walk along actin filaments with calmodulin.
What does each tropomyosin molecule have?
Seven evenly spaced regions with similar amino acid sequences, each of which is thought to bind to an actin subunit in the filament.
What pumps Ca2+ back into SR?
ATPase pump
Mechanics of excitation-contraction coupling. How does acethycholine work?
Acetylcholine opens gated channel, Na+ ions enter cell, depolarize membrane. Action potential throughout plasma membrane and T-tubules by voltage-gated Na+ channels. Dihydropyridine receptor activates ryanodine receptor, which releases Ca2+ into cytosol.
Difference between resting and activated neuromuscular junction.
Resting : resting nerve terminal, acetylcholine within vesicle at terminal. Voltage gated calcium channels closed, acethylcholine gated cation channel closed, voltage gated sodium channels closed, dihydropyridine receptor closed and ryanodine receptor closed. Activated: nerve impulse travels down axon to terminal. Acetylcholine neurotransmitter expelled via vesicle to cytosol, where it activates a channel dependent on acetylcholine, which opens. Sodium goes through channel and polarizes cell. Voltage gated calcium channel opens (L channel), opens ryanodine receptor so calcium ion leaves SR and goes into cytosol.
Four classes of muscle cell of a mammal
Heart muscle cell, smooth muscle cell, skeletal muscle cell, myoepithelial cell
What coordinates heart muscle contractions?
Gap junction connections between cells
Difference between heart muscle cells and skeletal muscle cells?
Heart muscles are single cells with single nucleus. Both have sarcomeres and striated appearances. Both joined by end to end junctions.
Smooth muscle cells
Not striated. Belong to connective-tissue family and are closely related to fibroblasts. Involuntary muscle. Actin/myosin, but doesn’t use troponin. Ca2+ activates calmodulin, wchih activates myosin light chain kinase, which regulates myosin assembly.
How is myosin dependent contraction regulated in smooth muscle/nonmuscle cells?
Not through troponin, but calmodulin and kinase.
How do calcium ion levels rise in smooth muscle?
Via signaling through GPCRs (peptide hormone signals) to increase Phospholipase C, IP3. Calcium ion channels on cell surface responde to voltage changes or chemical signals.
How does calcium ion work with calmodulin?
It activates calmodulin, which activates myosin light chain kinase, allowing it to phosphorylate myosin, promoting assembly of bipolar filaments
How do myosin and actin filaments slide?
Myosin filaments slide antiparallel arrays of actin filaments to contract the cell.
How does myosin light chain kinase (MLCK) work?
Controlled phosphorylation by MLCK of one of the two light chains on non-muscle myosin II has at lease two effects. It causes a change in the conformation of the myosin head, exposing actin binding site, and it releases the myosin tail from a sticky patch on the myosin head, thereby allowing the myosin molecules to assemble into short, bipolar, thick filaments.
How does listeria bacterial pathogen enter host cell and move inside it without myosin motor force?
By phagocytosis, and then it escapes the endocytic vesicle and takes advantage of actin assembly by host cell to move within cytosol. It nucleates actin and assembles actin tail that makes it motile.
What four purified proteins are needed to reconstitute listeria motility?
Actin, Arp 2/3 complex, capping protein, and cofilin.
What activates Arp 2/3 complex from bacteria?
The ActA protein on bacterial surface.
How do filaments grow from bacteria?
On their plus end until capped by capping protein.
What recycles actin?
Cofilin, which enhances depolymerization at the minus end of the filaments. Polymerization is then focused at the rear surface of the bacterium, propelling it forward.
Examples of cell migration and chemotaxis
Fibroblasts migrate in connective tissue; white blood cells move toward sites of bacterial infection; neural crest cells migrate during development; axonal growth cones are guided by extracellular signals; cancer cells migrate during metastasis; amoebae crawl toward a food source
Two types of signal molecules
Diffusible signals and nondiffusible signals such as extracellular matrix
What do signaling factors do?
Bind to receptors leading to activation of G-proteins in the Rho family, like Rac, Rho, Cdc42.
Three types of actin arrays in crawling cells
Contractile bundles, gel-like networks and tight parallel bundles
Contractile bundle
Example: stress fiber. Antiparallel arrangement of F-actin interspersed with myosin II filaments that contract the actin filaments
Gel-like network
Example: cell cortex. Network of F-actin cross-linked with filamin.
Tight parallel bundle
Example: filopodium. Parallel F-actin bundle cross-linked with Fimbrin crosslinks.
Three steps of crawling cell motility
Protrusion at the leading edge, attachment, contraction at the trailing edge
Components responsible for leading edge protrusion
Filopodium, lamellipodium, pseudopodium
Protrusion does what
Edge moves forward and stretches actin cortex. Actin assembly at leading edge “pushes” out membrane. Plus ends are oriented at plasma membrane.
During protrusion, what aids disassembly of F-actin at minus ends?
Cofilin.
What does actin network exhibit during protrusion?
Treadmilling
How does PM enlarge at leading edge?
Secretions of membrane vesicles enlarge PM.
How do monomers of actin disassembled get to plus end?
They diffuse
What does Arp2/3 complex do during protrusion?
It mediates nucleation at the front. Newly nucleated actin filaments are attached to the sides of preexisting filaments, primarily at a 70 degree angle.
Focal adhesions
Dynamic assemblies of structural and signaling proteins that link the migrating cell to the extracellular matrix
Contraction at trailing edge
Myosin II propels cell forward to relax some of the tension caused by attachment and traction
How does cell anchor to substrate?
Stress fibers (bundles of actin filaments in cells) interact with integrins at focal contacts
Where can stress fibers be anchored?
At one end or both ends by focal contacts
How can antiparallel actin filaments contract?
Via myosin II filaments
Integrins
Transmembrane heterodimers that link the extracellular matrix to the cytoskeleton.
Integrins made of
Composed of two noncovalently associated glycoprotein subunits called alpha and beta. Span cell membrane with short c-terminal tails and large n-terminal extracellular domains
What do focal adhesions interacting with stress fibers provide?
Attachments to the extracellular matrix
What interactions link actin cytoskeleton to the substratum?
Interactions between actin-binding adaptor proteins and integrins
Rho protein signals
Local regulation of actin cytoskeleton by external signals. Closely related monomeric GTPases thar are members of the Rho protein family – Cdc42, Rac, and Rho. Act as switches that cycle between an active GTP bound state and an inactive GDP bound state.
Cdc42
Activation of this protein on inner surface of PM triggers actin polymerization and bundling to form filopodia/ Fromin activation.
Rac
Activation of Rac promotes actin polymerization at the cell periphery, leading to the formation of sheetlike lamellipodial extensions (Arp 2/3 activation)
Rho
Activation of Rho promotes both the bundling of actin filaments with myosin II filaments into stress fibers and the clustering of integrins and associated proteins to form focal adhesions. Fromin activation.
How are derivatives of phosphotidylinositol important in signaling pathways?
Phosphoinositides derived through reversible phosphorylation. PI(3,4,5)P4 serves as docking site for various intracellular signaling proteins. PIP3 second messenger provides a docking site for proteins with pleckstrin homology domains (PH). Phosphorylation reversed by PTEN phosphatase.
Two pathways in which Rac-GTP creates branched actin web in lamellipodia?
a. WASp family / Arp 2/3 (branching nucleator)/ lamellipodia b. PAK / filamin (web cross-linker) / lamellipodia
Two pathways in which Rac-GTP leads to less stress fiber formation
a. PAK, inhibits MHC, which leads to decreased myosin activity, which leads to less stress fiber formation b. PAK inhibits MLCK, which leads to decreased myosin activity, which leads to less stress fiber formation
Two pathways through which Rho-GTP increases stress fibers and lead to integrin clustering and focal adhesion formation
a. Rho-dependent kinase (Rock) promotes LIM kinase, which inhibits cofilin/ Rock inhibits MLC phosphatase and promotes MLC(P), which leads to increased myosin activity, which leads to more stress fibers and focal adhesion formation b. Rho-GTP stimulates formins, which stimulate actin bundle growth, which stimulates more stress fibers and focal adhesion formation
in chemotactic signaling, where are G protein couple receptors found in dictyostelium and neutrophils?
On all sides of cell
In dictyostelium and neutrophils, where do signal molecules bind GPCR receptors?
On one side of the cell
What do G protein beta gamma subunits activate?
PI3 kinase
What does PI3kinase do when activated?
Converts PtdIns(4,5)P2 to PtdIns(3,4,5)P3 in the PM
What binds PI(3,4,5)P3?
Plekstrin homology (PH) domain proteins (like RacGEF)
What happens when Rac is activated?
Stimulates WASp, stimulates Arp2/3 assembly of actin networks to extend leading edge
What is PTEN?
A 3’ phosphatase
What does PTEN do in chemotactic signaling?
PTEN remains at the sides and back and keeps PI(3,4,5)P3 signal at the front
What molecules act as signals for dictyostelium?
Folic acid, cAMP
What molecules act as signals for neutrophils?
N-formylmethionine peptides
What rearranges during phagocytosis?
Actin cytoskeleton
What does phagocytic white blood cell wrap around bacterium?
Pseudopod grows out and around bacterium. Peudopod made of actin filament network.
