Lecture 20 Cell Organization and Movement I: Microfilaments

Cytoskeleton

Network of filamentous structures in cells.

Microfilaments

Filaments composed of polymerized actin monomers.

Actin

Highly conserved protein forming microfilaments.

G-actin

Monomeric form of actin with ATPase activity.

F-actin

Polymerized form of actin, helical structure.

Polarity of Actin Filaments

Distinct ends: (+) barbed and (−) pointed.

Nucleation Phase

Initial lag phase in actin filament formation.

Elongation Phase

Rapid assembly of actin subunits onto filaments.

Steady State Phase

No net change in filament length.

Critical Concentration (Cc)

Threshold for actin filament assembly.

Treadmilling

Dynamic assembly and disassembly of actin filaments.

Profilin

Like GEFs. Favors assembly and helps growth at (+)

Cofilin

Fragments ADP actin filament regions enhancing depolymerization by making more (-) ends.

Thymosin-β4

Buffer. Sequesters G actin at high concentrations, release G actin at low concentrations to polymerize.

Myosins

Actin-based motor proteins facilitating movement toward + end

ATP Hydrolysis

Promotes actin depolymerization after polymerization.

Actin Monomer Concentration

Determines filament formation and stability.

Actin Filament Structure

Two helically wound strands of actin.

Microtubule Organizing Centers

Structures organizing microtubules in cells.

CapZ

Limits actin assembly at the (+) end to that of the (-) end

Gelsolin

Severs actin filaments by inserting itself between actin subunits of the helix and blocks new (+) end.

Tropomodulin

Stabilizes actin filaments by capping (−) ends.

Formin FH2 Domain

Nucleates assembly of NEW actin filament

Arp2/3 Complex

Nucleates branched actin filament assembly.

Myosin I

Single-headed myosin with variable light chains.

Myosin II

Forms bipolar filaments, two heavy chains. generate contractile force in muscles.

Myosin V

Two heads, transports cargo along actin.

Powerstroke Mechanism

Predicts myosin step size relates to neck length. ADP- actin after release of Pi.

Skeletal Muscle Fibers

Multinucleated cells with ordered myofibrils.

Sarcomere

Contractile unit with myosin and actin filaments.

Nebulin

Determines length of thin actin filaments. RULER

Titin

Elastic protein stabilizing thick filaments. Prevents overstretching of sarcomere

Troponin

Regulates muscle contraction by binding Ca2+.

Tropomyosin

Prevents myosin binding to actin filaments.

Cytosolic Ca2+

Triggers muscle contraction via signaling pathways.

Lamellipodia

Extensions at cell leading edge for migration.

Focal Adhesions

Connections between actin cytoskeleton and ECM.

Chemotaxis

Movement directed by extracellular chemical cues.