Eukaryotic Test 3 Lecture 4

• 1. ~ 8nm in diameter
• 2. Building blocks: globular protein, actin
• 3. Actin monomers polymerize in presence of ATP to form flexible, helical filaement
• 4. Actin subunit has polarity, all subunits of microfilaments pointed in same direction, therefore microfilament has polarity

• Myosin fragment cleaved by proteolytic enzymes
• Binds to actin molecules all along the microfilament

When S1 are bound, 1 end of microfilament appears pointed (minus-slowgrowing) while other looks barbed (plus(+) fastgrowing)

Actin monomer binds to molecule of ATP before incorporating into a filament

Initial stage of MF formation (nucleation) occurs slowly whereas subsequent stage of filament elongation occurs rapidly

Addition of preformed actin filaments can bypass nucleation stage

Convert chemical energy into mechanical energy, which is used to move cellular cargo attached to the motor

3 broad families: kinesins, dyneins, myosins,

Motor proteins move unidirectionally along cytoskeletal track in stepwise manner

Kinesins: Plus (+) end-directed microtubular motors that have a tetrameric structure

Globular head (motor domain): Binding molecule, ATP-hydrolyzing, force-generating engine. Motor domain is strikingly similar structure to that of myosin

Fan-Shaped Tail: Binding cargo

Kinesin molecules move along protofilaments of microtubules at velocity proportional to ATP concentration

Composed of two identical heavy chains and a variety of intermediate and light chains

1. A force-generating agent in the positioning of the spindle & movement of chromosomes during mitosis

2. A Minus (-) end-directed microtubular motor for the positioning of golgi complex and movement of organelles, vesicles, & particles thru the cytoplasm

Kinesin and dynein move vesicles in opposite directions along the same microtubule

Molecular motors move toward the Plus (+) end of an actin filament.

Share characteristic motor head which includes an actin filament binding site & an ATP binding site& a highly divergent tail domain

• 2 broad groups:
• 1. Conventional (Type II) myosins: first identififed in muscle cells

2. Unconventional myosins: >17 differnt classes

• 1. Pair of globular heads (contains all machinery required for motor activity)
• 2. A pair of necks, each consisting of a single, uniterrupted (A)-helix and 2 associated light chains
• 3. A single, long, rod-shaped tail formed by the intertwining of long (A)-helical sections of the 2 heavy chains to form an (A)-helical coiled coil

Fibrous tail portion plays a structural role, allowing proteins to form filaments.

Ends of the tails point to the center of the filament and globular heads point away from center, forming a bipolar structure.

• 1. Has a long neck (23nm, 3X myosin II)
• 2. Each step covers 13 actin subunits (36nm)
• 3. Walk along catin filament by a "hand-over-hand" model

Hairlike motile organelles that project from surface of eukaryotic cells. Essentially the same in structure

Prokaryotic Flagella are simple filaments w/ no evolutionary relationship to eukaryotic ones. Bacterial flagella consist largely of flagellin ( a protein)

Tend to occur in large #'s on a cell's surface.

Beating activity usually coordinated & move cell in a direction perpendicular to cilia

Exhibit a variety of different beating patterns (waveforms)

1. Membrane: continuous with the PM of the cell

2. Axoneme: core of the cilium, which contains an array of MTs that runs longitudinally thru entire organelle

Axoneme has a "9+2" array of Microtubular structure

A structure similar to a centriole from which a cilium or a flagellum emerges

The machinery for ciliary and flagellar motion resides within the axoneme

Kinesin II moves building materials to the assembly site at the tip of the growing axoneme

Dynein 1b transports Kinesin II and recycled axonemal proteins back to the basal body