Ch. 16 Cytoskeleton

What is a microtubule?

• originate near nucleus

• every one made of tubulin

• outer membrane 25nm diameter

• MTOC: microtubule organizing center; centrosome

What is a centrosome?

• pair of centrioles in center

• spherical complex of proteins

• surface of sphere: gamma-tubulin ring

How many proteofilaments make up a microtubule?

13

What is necessary to polymerize a microtubule?

GTP cap

• GTP bound

• binds to positive end of filament

• tubulin binds to GTP, a and b tubulin spontaneously dimerize

What does the gamma-tubulin ring do?

nucleating sites

• foundation of every microtubule (-) end to gamma ring

What are the two associated proteins with microtubules?

both ends can bind to microtubules

non-motor MAPs regulate MT stability, targets of protein kinases, phosphorylation alters binding = microtubule falls

TAU (tubulin-associated unit)

• stabilizing, coating entire microtubule

MAP2 (microtubule-associated protein 2)

• coating entire microtubule

What happens in Alzheimers?

1. Beta amyloid plaque build up

2. Tau-Neurofibrillary tangles = hyper-phosphorylated TAU proteins

What are the motor proteins in microtubules?

mediate proteins and vesicular transport; every step requires ATP

• kinesin and dynein

What is kinesin? What is dynein?

• anterograde transport that moves towards positive end (GTP cap)

• retrograde transport that moves towards negative end

What is the axoneme?

• fills core of every cilium and flagellum → 9+2 arrangment

• lots of stabilizing proteins that prevent doublets from sliding past one another

• bending due to linking proteins = curvature

• incomplete microtubule = 11 protofilaments

What are ciliary dynein? What defect can happen?

specific to ciliary/flagellum

• Immotile Cilia Syndrome: defect in ciliary dynein, cilia not mobile, male infertility, respiratory tract infection = cannot move mucus

What are actin filaments?

• polymerized in different orientations

• made of actin

• 7 nm diameter

• concentrated in cortex (outer area)

Actin filaments in intestinal epithelial cells

• located in cell cortex beneath PM

• support microvilli to increase surface area for absorption

• microvilli = actin, cilia/flagella = microtubules

• contractile properties: motor proteins (actin- thin and myosin-thick)

• organized into sarcomeres in muscle cells for contraction

What is the actin monomer?

• globular (G actin) in cytosol

• ATP hydrolysis occurs after polymerization

• filamentous (F-actin) length does not change

• minus end actin monomer falls off bound to ADP

What is an intermediate filament?

• “spiders nest in cell”

• tissue-specific proteins: keratin is IF for epithelium

• 10-12 nm diameter

• structural framework, stable

• resist mechanical stress

What is the structure of intermediate filament?

• 8 protofilaments = intermediate filament = 16 coiled coil dimers

• staggered tetramer of 2 coiled coil dimers

Major types of proteins in vertebrate cells?

nuclear: lamins located in nuclear lamina

vimentin-like: desmin located in muscle

epithelial (skin): keratins located in epithelial cells