Ch 17 Microtubules

How do microtubules affect organelle positioning?

they help position membrane-enclosed organelles within the cell

What is the main organizing role of microtubules in eukaryotic cells?

they form long, stiff tubes that organize the cytoplasm and serve as tracks for vesicle and organelle transport

Are cytoplasmic microtubules permanent structures?

no, they can rapidly disassemble and reassemble depending on cellular needs

What happens to microtubules during mitosis?

cytoplasmic microtubules disassemble and re-form the mitotic spindle

Besides dynamic tracks, what stable structures can microtubules form?

cilia and flagella

What subunits are microtubules built from?

tubulin dimers

What two proteins form the tubulin dimer?

α-tubulin and β-tubulin

How are α-tubulin and β-tubulin held together?

by tight noncovalent interactions

What is a protofilament?

a linear chain of alternating alpha and beta tubulin dimers

How many protofilaments make up a microtubule?

13 around a hollow center

Which end of the microtubule has exposed β-tubulin?

the plus end

Which end has exposed α-tubulin?

the minus end

Tubulin adds faster to the

plus end (beta)

Tubulin dimers bound to GTP add to the

plus end (beta)

GTP is hydrolyzed to

GDP

• this destabilizes the interaction between dimers

A GDP-bound tubulin dimer will dissociate from the tubule IF

no other GTP tubulin dimers have been added after it

If free GTP tubulin concentration is HIGH

the association rate is faster than dissociation rate

• the microtubule extends

If free GTP tubulin concentration is LOW

the dissociation rate is faster than association rate

• the microtubule shrinks

What is the main microtubule-organizing center in many animal cells?

the centrosome

The microtubules in neuronal axons are

stable

The microtubules in neuronal axons are positioned with the

plus end toward the axon terminal

Motor proteins transport neurotransmitter-containing vesicles through

the axon using microtubules as tracks

What drives saltatory movement of organelles?

motor proteins using ATP hydrolysis

Saltatory movements occur along which cytoskeletal elements?

microtubules or actin filaments

What ensures motor proteins move in a single direction along filaments?

stereospecific binding of the motor’s heads to the filaments

What provides energy for motor protein movement?

ATP hydrolysis

• this process is energy dependent

What are the major families of microtubule motor proteins?

• kinesins

• dyneins

Which direction do most kinesins move along microtubules?

toward the plus end

• usually outward from the cell body

Which direction do cytoplasmic dyneins move along microtubules?

toward the minus end

• toward the cell body

What is the general structure of most kinesins and cytoplasmic dyneins?

dimers with two globular ATP-binding heads and one tail

What determines the cargo carried by a motor protein?

the motor proteins tail

• it binds specific vesicles/organelles

Where is the ATPase activity located in kinesin and dynein?

in their globular heads

How do motor proteins move along microtubules?

by ATP-driven conformational changes enabling “hand-over-hand” stepping

The main function of motor proteins in most cell types is the

transport of vesicles

• intracellular traffic

Dynein always uses

adaptor proteins

What stabilizes some microtubules so they do not show dynamic instability?

association with microtubule-associated proteins

What are cilia?

hairlike, membrane-covered structures extending from eukaryotic cells

What structure organizes the microtubules in a cilium?

the basal body

How do flagella operate differently from cilia?

they’re longer and move the entire cell rather than fluid

What causes movement in cilia and flagella?

bending from microtubules sliding against one another

Which motor protein generates bending in cilia and flagella?

a specialized form of dynein

How is dynein positioned on microtubules in cilia/flagella?

• tail is attached to one doublet (tubule A)

• heads interact with adjacent doublet (tubule B)

What are the main roles of motile cilia?

• moving fluid over cell surfaces

• moving cells through fluid

Nexin =

the linking protein that anchors microtubules