BIOL 212 Lecture Notes - Lecture 28: Sliding Filament Theory, Endoplasmic Reticulum, Atp Hydrolysis
18 May 2018
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Lecture 28 Notes
Actin Filaments (microfilaments)
- Used for many cellular functions:
o Contractile ring in cell division
o Contractile bundles (cell shape change)
o Microvilli (diff from cilia)
o Lamellipodia and filopodia in migrating cells
- Have inherent polarity, like microtubules
- Carry ATP
- Monomers are usually (not always) added to + end
- ATP is hydrolyzed soon after monomer joins filament
- Undergoes “treadmilling”
- Actin binding controls filament structure and dynamics
- Actin polymerization at the leading edge allows a cell to move
o Specialized actin-based structures propel leading edge forward (lamellipodium/filopodium)
Nucleated w/help ARP (Actin Related Protein)
- ARP complex helps build branched actin filament structures
Myosins are Motor Protein that Move Along Actin Filaments
- Using energy of ATP hydrolysis, myosins walk toward plus end actin filaments
- Myosin-II molecules can associate to form myosin filaments
- Bipolar myosin-II molecules can slide actin filaments and shorten an actin filament bundle, mechanism
forms basis for cellular contractility
Skeletal Muscle
- Consists of myofibrils that are divided into sarcomeres
- Mature muscle cell is a syncytium (many nuclei w/in single plasma membrane)
- Sarcomeres are contractile units of muscles
- Muscles contract by sliding filament mechanism
- Myosin walks along actin in 4 step cycle
o Release
o Cocking
o Attachment
o Power stroke
- T tubules and sarcoplasmic reticulum surround the myofibrils in muscle cells
- Sarcoplasmic reticulum stores a large amount of Ca2+, lumen of T tubule is continuous w/EC space
- Increase in cytosolic Ca2+ concentration allows access of myosin heads to actin filaments
- Tropomyosin blocks myosin binding site, when Ca2+ added myosin-binding site exposed, troponin-
complex-dependent tropomyosin movement
Cell Signaling
- Cytoskeleton can be reorganized by cell signaling
- Actin polymerization can be regulated by cell signaling
- Activation of GTP-binding proteins can affect the organization of the actin cytoskeleton
o Rho activation
Slime Mold (amoeba) Dictyostelium
- Cells can sense a signal, reorganize the cytoskeleton and move in the direction of the signal
- Cells can sense a 2-10% signal concentration difference b/t the front and back end
- Response is rapid
- Response is reversible
Cell Migrate in Particular Direction
- Directional sensing → setting cell polarity → formation of pseudopod; reorganization of the
cytoskeleton → movement
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Document Summary
Lecture 28 notes: contractile ring in cell division, contractile bundles (cell shape change, microvilli (diff from cilia, lamellipodia and filopodia in migrating cells. Monomers are usually (not always) added to + end. Atp is hydrolyzed soon after monomer joins filament. Actin binding controls filament structure and dynamics. Actin polymerization at the leading edge allows a cell to move: specialized actin-based structures propel leading edge forward (lamellipodium/filopodium) Arp complex helps build branched actin filament structures. Myosins are motor protein that move along actin filaments. Using energy of atp hydrolysis, myosins walk toward plus end actin filaments. Myosin-ii molecules can associate to form myosin filaments. Bipolar myosin-ii molecules can slide actin filaments and shorten an actin filament bundle, mechanism forms basis for cellular contractility. Consists of myofibrils that are divided into sarcomeres. Mature muscle cell is a syncytium (many nuclei w/in single plasma membrane) Myosin walks along actin in 4 step cycle: release, cocking, attachment, power stroke.
