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): 16 80-89% (58-64): 7 70-79% (50-57): 9 60-69% (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.
I. Microtubules Structure and assembly Dynamic instability model Microtubules in motion II. Microfilaments Video of MF-based motility 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. Outline/Learning Objectives
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 (a) killer lymphocytes acting on infected cells (b) removal of survival factors/ DNA damage • 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.
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 • Interphase microtubules: dynamic instability • Chromatophores: pigment granule transport along microtubules • Axopodia in Heliozoans: prey capture and transport along microtubules • 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.
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