Announcements

• Please remove exam answer sheets, homework from your folders.

• News story on “semi-identical twins” posted on Blackboard.

• Today, April 4: microtubules, pp. 425-437• Friday, April 6: Microtubules and

microfilaments, pp. 425-446 (742-762 in 5th Ed.)

• Monday, April 9: Muscle contraction, Cell motility, intermediate filaments.

Exam 3 Results

• Key and full standings posted outside BR 179• Average before curve = 45/72 = 63%• Average after curve = 58/72 = 80%

– 13 points added to everyone’s raw score to curve• High = 87/72 (120%)

90-100% (>=65): 1680-89% (58-64): 770-79% (50-57): 960-69% (43-49): 9<60% (<43): 5

• Do add up your raw score, count the number correct, multiply by 3, add any extra credit then add 13 points.

Outline/Learning Objectives

I. MicrotubulesA. Structure and

assemblyB. Dynamic instability

modelC. Microtubules in motionII. Microfilaments

A. Video of MF-based motility

B. Actin and assembly

After reading the text, attending lecture, and reviewing lecture notes, you should be able to:

• Compare and contrast microtubules, microfilaments and intermediate filaments.

• Describe microtubule structure and assembly/disassembly, including nucleation and dynamic instability. Explain the function of MAPs, kinesin, and dynein.

• Explain how axonemal structure determines ciliary movement.

• Describe microfilament structure and assembly.

• Give examples of microfilament-based motility.

Semi-identical twinsSouter et al. 2007

RSTK Pathway

• Another class of growth and differentiation factors signal through receptor serine-threonine kinases.

• E.g. TGFβ• Many involved in

development• Smad transcription

factors become active when phosphorylated.

Steps of apoptosis, or programmed cell death

• Examples: normal cell senescence, damaged/infected cells, embryonic digits, tadpole tails, pruning of infant neurons.

Apoptosis Pathway

• Programmed cell death discovered in nematode C. elegans.

• Present in other organisms, e.g. mammals.

• Triggered either by presence of death factors or removal of survival factors.

(a) killer lymphocytesacting on infected cells

(b) removal ofsurvival factors/DNA damage

Components and Functions of the Cytoskeleton

Recent review: Pollard, T.D. (2003). Nature 422:741-745.

Self-assembly of cytoskeletal filaments

• Polymerization does not involve covalent bonds, rather weak interactions between monomers.– Therefore, can assemble or disassemble rapidly,

without covalent bond formation or breaking.

• Combine strength with adaptability through lateral interactions in addition to length-wise interactions.

• Nucleation is rate-limiting step of polymerization

Microtubules

• Cytoplasmic MTs:– dynamic growth and

collapse– in nerve processes,

mitotic spindles, elongating cells, etc.

• Axonemal MTs:– stable– in cilia, flagella

• http://raven.zoology.washington.edu/celldynamics/

Videos: Microtubules in Motion

1. Interphase microtubules: dynamic instability

2. Chromatophores: pigment granule transport along microtubules

3. Axopodia in Heliozoans: prey capture and transport along microtubules

4. Mitosis in animal, plant cells

Microtubule Structure

• 25 nm outer diameter• 12 protofilaments, polymers of

heterodimeric and -tubulin (50 kD each). -tubulin binds GTP

• Destabilized by:– cold, pressure– Drugs colchicine, nocodazole, etc.– > 10-6 M Ca2+

• Stabilized by:– Drug taxol– MT-associated proteins (MAPs)

Centrosomes organize cytoplasmic MTs

• Animal centrosomes contain two centrioles (9 short triplet MTs), plant centrosomes don’t.

• Pericentriolar material nucleates MTs (animals and plants) (gamma)-tubulin– pericentrin

• Minus end of MT starts in centrosome, plus end is away.

Microtubule organization in cells

Dynamic Instability of MTs:

GTP cap stabilizes + end

of MTs

Recent review: Howard and Hyman. (2003). Nature 422:753-758.

Microtubule Poisons(and Anti-Cancer Drugs)

• Colchicine– Alkaloid from meadow saffron– Binds tubulin monomers, prevents

assembly processes that require assembly are blocked

– Colcemid, nocadazole, etc. also (-) assembly

• Taxol– Alkaloid from Pacific yew tree,

effective chemo- in ovarian cancer– Can now be synthesized in vitro– Binds microtubules, stabilizes,

prevents disassembly processes that require disassembly are blocked