Eukaryotic Test 3 Lecture 3

• 1. Dynamic scaffold that provide structural support & shape
• 2. Internal framework responsible for positioning of organelles
• 3. Network of tracks that direct movement of materials & organelles
• 4. Force-generating apparatus that moves cell from 1 place to another
• 5. Essential component of cell's division machinery

• Hollow, tubular structures. components of diverse cell structures
• Outer Diameter: 25nm, Wall thickness: ~4nm
• Composed of globular proteins (protofilaments) arranged in longitudinal rows. Each tubule consists of 13 protofilaments

• Globular protein units: (A)-tubulin, (B)-tubulin
• Heterodimer: assembly unit w/polarity
• Protofilament: Linear array of heterodimers in same polarity
• Microtubule: 13 protofilaments aligned side by side in same polarity

• Plus (+) End: Terminated by a row of (B)-tubulin subunits
• Minus (-) End: Terminated by a row of (A)-tubulin subunits

Function as cross bridges connecting microtubules to each other to maintain parallel alignment. Increase stability and promote assembly of microtubule

1 domain attach to side of microtubule, other projecting outward as a filament form mt's surface

Microtubule binding activity of MAPs regulated by phosphorylation

• Requires a GTP molecule to be bound to (B)-tubulin subunit
• 1. GTP-dimers added to plus (+) end of MT
• 2. Open end closes and GTP hydrolyzed to GDP
• 3. GDP-tubulin less able to fit into straight protofilament, which destabilizes MT
• 4. Strain is released as protofilaments curl outward from plus (+) and undergo depolymerization.

**GTP-hydrolysis makes MT inherently unstable. MT's disassemble soon after formation in absence of stabilizing factors (MAPs)

Microtubules nucleated by centrosomes: contain 2 centrioles

Centrioles: Surrounded by amorphous, electron-dense Pericentriolar Materials (PCM)

~0.2 um in diameter & twice as long and contain 9 evenly spaced fibrils

Each fribril consists of 3 MTs: A, B, C. A is only complete.

MTs connected to center by a radial spoke

Always exists in pairs at right angles

Basal Body: resides at base of cilium or flagelum. Idential in structure to centrosomes, giving rise to one another

Plant cells lack centrosomes/centrioles, but have proteins homologous to those of centrosomes

Centrosomes are at the center of microtubular network

MTs nucleated at centrosomes, & subunits are added at Plus (+) End of polymer away from the centrosome

Control # of MTs, polarity, # of protofilaments, & time/location of assembly

• MTOCs initiate nucleation of MTs:
• --Insoluble fibers of PCM serve as attachment sites for ring shaped structure having same diamter as MTs and contain Y-tubulin

Y-TuRCs (Y-tubulin ring complexes): Helical array of Y-tubulin subunits forming an open, ring-shaped template on which 1st row of (AB)-tubulin dimers assemble

Only (A)-tubulin of heterodimer can bind to ring of Y-subunits. Therefore Y-TuRCs determine polarity of ENTIRE MT & form a cap at its minus (-) end.

• 1. Interphase: MTs distributed throughout cortex
• 2. MTs disappear from most of cortex as cell nears mitosis. Leaves a single transverse preprophase band
• 3. Preprophase band is lost & MTs reappear as mitotic spindle
• 4. After separation of chromosomes, mitotic spindle disappear and replaced by bundle of MTs (phragmoplast)

• Induced by:
• 1. Cold temperature
• 2. Hydrostatic pressure
• 3. Elevated Calcium (Ca2+) concentration
• 4. Chemicals: colchicine, vinblastine, vincrisitine, nocodozale, & podophyllotoxin

Taxol binds to MT polymer inhibiting disassembly. Thereby preventing cell from assembling new MT structure.

Growing and shrinking of MT can coexist in same region of cell.

A given MT can switch between growing and shrinking phases unpredictably

Allows cells to respond rapidly to changing conditions requiring remodeling of MT cytoskeleton

Strong, ropelike fibers providing mechanical strength to cells subjected to physical stress (neurons, muscles & epithelial cells)

~10 nm in diameter

Only in animal cells

Polypeptides contain central, rod-shaped, (A)-helical domain of similar length & homologous AA sequence.Domain flanked on either side by globular domains of various sizes and sequence

(A)-helical rods of 2 polypeptides wrap around each other to form a dimer ~45nm in length. Dimer has polarity

• Two dimers become aligned in staggered fashion w/ N- and C-terminus in antiparallel directions. Tetramer lacks polarity
• Tetramer associate w/1 another both side to side & end to end to form poorly described intermediates that assemble into final filament

IFs are less sensitive to chemical agents and more difficult to solubilize.

Assembly & disassembly of most IFs are controlled by phosphorylation of subunits