BIO 211 Lecture 9 Microtubules, Intermediate Filaments, and Septins

dynamic

microtubules are highly —

plus (+)

microtubules’ — ends point towards the cell periphery

cargo

Microtubules help move — and signaling proteins around the cell

static

Intermediate filaments are relatively —

• take much longer to remodel

intermediate filaments

— are non polarized

strength

Intermediate filaments provide mechanical — to cells and tissues

barriers

Intermediate filaments and septins form physical —

stabilizes

Taxol — filaments

• binds along filaments

• prevent dynamic instability

taxol

—, a compound found in yew trees, stabilizes microtubules

• prevent dynamic instability

nocodazole

a compound that depolymerizes microtubules by binding tubulin subunits

depolymerizes

nocodazole — microtubules by binding tubulin subunits

• synthetic

depolymerizes

colchicine — microtubules by capping filament ends

• autumn crocus

colchicine

— depolymerizes microtubules by capping filament ends

• autumn crocus

heterodimer, noncovalent

The tubulin subunit is a — formed from a-tubulin and b-tubulin, which are connected by — bonds

13

A microtubule is a hollow cylindrical structure built from — parallel protofilaments, each composed of α/β-tubulin heterodimers stacked head to tail and then folded into a tube

lateral

Microtubules also have — connections

• The multiple contacts among subunits make microtubules stiff and difficult to bend.

a-tubulin

The GTP that is bound to — is physically trapped at the dimer interface and is never hydrolyzed or exchanged; it can therefore be considered to be an integral part of the tubulin heterodimer structure

B-tubulin

— may be bound to either GTP or GDP

• This difference is important for microtubule dynamics.

GTP cap

When the next heterodimer is being added before enough time has gone by for GTP hydrolysis to occur on the previous heterodimer, a — is formed

catastrophe

microtubule switches from growing to shrinking

Rescue

microtubule switches from shrinking to growing

protection

GTP cap acts as — to the GDP dimers deeper in the filament

catastrophe

loss of GTP cap leads to —

rescue

Regain of GTP cap leads to —

free ends

Microtubule only falls apart at the —

Beta

the exchangeable GTP is on the — subunit of Tubulin

GTP hydrolysis

— causes mechanical strain on the noncovalent bonds between the filaments

• Heterodimers don’t fit together as well

GDP, GTP

Dissociation of a — bound subunit is more favorable than the dissociation of a — bound subunit because the free energy change is more negative when it dissociates

free energy

Dissociation of a GDP bound subunit is more favorable than the dissociation of a GTP bound subunit because the — change is more negative when it dissociates

gamma tubulin

— ring complex acts like Arp2 and Arp3, providing a template of 7 tubulin building blocks in the correct orientation so that alpha tubulin heterodimers can be added

microtubule organizing center (MTOC)

Microtubules are generally nucleated from a specific intracellular location known as a — where γ-tubulin is most enriched

Microtubule organizing center (MTOC)

Microtubules all emanate from the —

minus (-)

— ends of microtubules are found at the center of MTOC

plus (+)

— ends of microtubules radiate outwards from MTOC

Gamma tubulin (y-tubulin)

 

— anchors the microtubule to the microtubule organizing center and prevents it from depolymerizing at the (-) end

Y-TuRC (Gamma tubulin ring complex)

acts as a template for microtubule formation by providing a scaffold that mimics the plus-end of a microtubule.

• Amplify microtubule polymerization

Stathmin

binds to 2 α/β-tubulin dimers, prevents their addition to growing microtubule

• prevent microtubule polymerization

• decreases the effective concentration of tubulin subunits that are available for polymerization

• enhances the likelihood that a growing microtubule will switch to the shrinking state

Kinesin-13

Catastrophe factor

Binds to GTP cap at + end of microtubule and converts ATP hydrolysis power to rip apart GTP cap

• reveal GDP bound heterodimers and filament falls apart

XMAP215

• binds to GTP cap and accelerates addition of heterodimers to the + end

• binds free tubulin subunits and delivers them to the plus end of a microtubule

• dramatically accelerates polymerization

+TIPS

promote microtubule growth by stabilizing and protecting the growing plus end

• amplify microtubule polymerization

connect + end to plasma membrane

plectin

acts as a linker protein that connects intermediate filaments (like keratins, vimentin, or desmin) with microtubules, actin filaments, and membranes.

kinesin

microtubule based motor

• move cargo such as organelles and macromolecules within the cell towards the plasma membrane

Cytoplasmic dynein

a microtubule motor that binds cargo on one end and the microtubule on the other end

• converts chemical energy into energy to make it walk from the + end of microtubule to the - end

•  generally transports cellular cargo towards the microtubule-organizing center (MTOC), which is the opposite direction from most other motor proteins like kinesins

minus (-)

Dynein moves toward the — end of the microtubule

Dynactin

a multi-protein complex that works in conjunction with dynein to regulate intracellular transport along microtubules towards the MTOC

Arp1

The filamentous backbone of the dynactin complex, forming a structure that acts like an actin filament. It is essential for cargo binding and cargo movement along microtubules.

Lamina

Nuclear intermediate filaments are composed of —

nuclear lamina

Nuclear intermediate filaments are found in the —

vimentin

a key intermediate filament protein that plays a central role in maintaining the structural integrity and mechanical stability of cell

• found in mesechymal cells and fibroblasts

mesenchymal, fibroblasts

Vimentin is found in cells of — origin and —

keratins

Epithelial intermediate filaments are composed of —

polarized

Intermediate filament monomers and dimers are —

nonpolar

An intermediate filament tetramer is —

• Both ends are chemically identical (no + and - end)

8

a lateral association of — tetramers give intermediate filaments their mechanical strength

plectin

helps attach intermediate filament bundles to adhesive structures at the plasma membrane

•  links the intermediate filaments to microtubules, actin filament bundles, and filaments of the motor protein myosin II

outer

the KASH domain is located in the — nuclear membrane

inner

The SUN domain is located in the — nuclear membrane

nuclear lamina

The KASH and SUN domain proteins connect the — to the cytoskeleton

septins

GTP-binding proteins called — serve as an additional filament system

• assemble into nonpolar filaments that form rings and cage-like structures, which act as scaffolds to compartmentalize membranes into distinct domains or to recruit and organize the actin and microtubule cytoskeletons.

diffusion barriers

Septins can form — in dividing yeast cells and at the base of cilia

septin

In primary cilia, a ring of — filaments assembles at the base of the cilium and serves as a diffusion barrier at the plasma membrane, restricting the movement of membrane proteins and establishing a specific composition in the ciliary membrane

plus (+)

Kinesin typically moves toward the — end of microtubules

minus (-)

Cytoplasmic dynein moves toward the — end of microtubules

larger

Cytoplasmic dynein is much — than kinesin

cell membrane

Kinesin is primarily involved in transporting organelles, vesicles, and proteins toward the —

nucleus

Cytoplasmic dynein is responsible for moving cargo toward the —, positioning organelles, and participating in mitotic spindle organization

dynactin

Cytoplasmic dynein requires —, a large protein complex, to effectively bind cargo and regulate movement.

crowded

regulatory/trafficking mechanisms are needed in the cell because the cytoplasm is very —

genes

alpha and beta tubulin are transcribed from different —

molecular timer

When a tubulin heterodimer binds to the plus end of the microtubule, a — is started

• GTP is then hydrolyzed to GDP

beta

The end of the + end of the microtubule is always composed of — tubulin

GTP cap, grows

when tubulin dimers are added to the + end of the microtubule faster than GTP hydrolysis occurs, a — forms and the microtubule —

shrinks

when tubulin dimers are added to the + end of the microtubule faster than GTP hydrolysis occurs, the GTP cap is lost and the microtubule —

decrease

The GTP cap is lost when there is a — in available heterodimers to be added

GTP cap

The — holds the microtubule together at the + end

nucleation

Y-tubulin’s only job is —

longer

The lag phase for a microtubule to form in vitro is — than an actin filament’s lag phase because 7 tubulin heterodimers need to come together, compared to 3 actin monomers

MTOC

Y-tubulin is enriched at the —

golgi

The MTOC is close to the — because vesicular traffic can hop on the - end

kinesin-13

— allows the microtubule to depolymerize even at a high concentration of tubulin heterodimers where polymerization is favorable

XMAP215

— allows the microtubule to polymerize even at a low concentration of tubulin heterodimers where polymerization is unfavorable

kinesin-1

Has a motor domain, and the other end is bound to a vesicle

• walks from - to +

• Hydrolyzes ATP to give chemical energy for conformational changes needed to walk along a microtubule

endocytic vesicles

cytoplasmic dynein can transport — from the + end to - end

structural strength

vimentin, lamins, and keratins all provide —

staggered tetramer

polarity of the intermediate filament is lost at the — conformation stage

contact points

increased number of — between subunits give intermediate filaments their structural strength and allow them to stretch

septins

in budding yeast, — wrap around the neck of the bud to control the traffic in and out of the bud

septins

in primary cilia, — are enriched at the base to restrict the passage of cytoplasm components into the cilia