Microtubules

Microtubule function

1. Structure/organization

2. Dynamics

3. Associated proteins

4. Locations of assembly

5. Motors

Singlet, doublet, triplet

Microtubules may assemble as ______, ______, or _______ tubule structures

Singlet microtubule

A single microtubule cylinder composed of 13 protofilaments

Doublet microtubule

two microtubules arranged in a parallel fashion (typically in cilia and flagella) that share 3 protofilaments

3

How many protofilaments does a doublet microtubule share

Triplet microtubule

three microtubule structure and are typically found in structures like the basal bodies and centrioles

heterodimeric

Tubulin subunits are ________

Tubulin Heterodimer

A complex consisting of one α-tubulin and one β-tubulin subunit that polymerizes to form the microtubule.

alpha-tubulin

A subunit of the tubulin heterodimer that has a GTP binding site but doesn’t have any GTPase activity

beta-tubulin

A subunit of the tubulin heterodimer that has a GTP binding site and GTPase activity

Protofilament

Linear chains of tubulin dimers arranged end to end. Microtubules are made of 13 of these that form a cylindrical structure.

plus end

the ______ has the beta tubulin facing out

Microtubule Polarity

Microtubules have directionality, with a plus end (faster growing) and a minus end (slower growing). The minus end is typically anchored in the Microtubule Organizing Center (MTOC).

Dynamic Instability

The characteristic behavior of microtubules where they alternate between growth and shrinking phases

GTP Cap Model

The model explaining dynamic instability, where GTP-bound tubulin at the plus end stabilizes the microtubule, and GDP-bound tubulin leads to depolymerization.

Catastrophe

Phase where microtubule shrinks very quickly

Colchicine

A drug that binds to tubulin and prevents its polymerization into microtubules, commonly used to disrupt microtubule dynamics in cells.

Taxol

A drug that stabilizes GDP-tubulin and prevents depolymerization

Microtubule Organizing Center

A cellular structure (such as the centrosome) that organizes microtubules, acting as a focal point for microtubule nucleation and anchoring.

membrane bound

The centrosome is NOT __________

Centrosome

The main Microtubule Organizing Center (MTOC) in animal cells, consisting of two centrioles and the surrounding pericentriolar material, responsible for organizing microtubules.

Pericentriolar Material

The material surrounding the centrioles in the centrosome, which contains γ-tubulin and other proteins essential for microtubule nucleation

Dendrite

For nerve cells, from the cell body to ________, there are both kinds of microtubule polarity found which is not usually the case

gamma-Tubulin Ring Complex (gamma-TuRC)

A protein complex found at the MTOC that acts as a template for microtubule nucleation, facilitating the polymerization of new microtubules.

nucleate

Centrioles do not directly ________ microtubules, it’s usually gamma TuRC in the PCM

Centrosome

The _____ is compromised of centrioles and the PCM (including the gamma tubulin)

9+0 arrangment

Centrioles have this arrangment of triplets

fluorescence recovery after photobleaching

Using ________, we can see that microtubules are dynamic structures

Recovery via microtubule dynamics

Mechanism by which fluorescence “recovery” might occur where first a catastrophe occurs from bleaching, then recovery/rescue

Recovery via new microtubule growth

Mechanism by which fluorescence “recovery” might occur where a brand new microtubule grows from the centrosome

Recovery via translocating microtubule

Mechanism by which fluorescence “recovery” might occur where a microtubule detaches from the centrosome (gamma TuRC), and gets transported along other microtubules by microtubule motor proteins

MAPs (Microtubule Associated Proteins)

A family of proteins that interact with microtubules to regulate their stability and dynamics. Examples include MAP2, Tau, and Katanin.

MAP2, Tau

Microtubule-stabilizing proteins that can prevent disassembly

Katanin

Microtubule-destabilizing protein that severs and promotes disassembly

Axon

Tau localization in a neuron

Cell body/dendrites

MAP2 localization in a neuron

protein arm length

The spacing of MTs depends on the MAP _____________

further apart

The overexpression of MAP2 leads to MTs being _________

closer

The overexpression of Tau leads to MTs being _________

2

A single MAP2 protein has __ MT binding sites

Microtubule Motors

Motor proteins that transport cellular cargo along microtubules. Kinesin moves towards the plus end, while dynein moves towards the minus end of microtubules.

Kinesin

Plus-end directed microtubule motor protein

Dynein

Minus-end directed microtubule motor protein

In vitro motility assay (“Bead Assay”)

A laboratory technique used to study the movement of motor proteins, where beads are coated with motor proteins and moved along microtubules to analyze their transport properties. (in ATP buffer)

Motor Processivity

The ability of motor proteins like kinesin and dynein to move continuously along microtubules without dissociating, which is crucial for efficient cargo transport.K

Kinesin structure

dimer with two heads (ATPase motor domains) that bind to microtubules, a flexible neck that allows movement, a stalk that connects the motor heads, and a tail that binds cargo. It walks in hand-over fashion

Dynactin

Adaptor protein that helps Dynein interact with membrane-bound cargo

Dynein heavy chain power stroke

The conformational change in the dynein motor protein triggered by ATP hydrolysis, which generates the force required for dynein to "walk" along microtubules.

Long-range, short-range

In animal cells, microtubule function for _____ transport while actin for _________

Motile and Immotile Cilia and Flagella

Cilia and flagella are cellular appendages that can be motile (move, to propel the cell or fluid) or immotile (stationary, for sensing or signaling purposes)

Nexin

A protein that links adjacent doublet microtubules in cilia and flagella, contributing to their bending motion during movement.

Axoneme

The core structure of cilia and flagella, composed of microtubules arranged in a 9 + 2 pattern (9 doublet microtubules surrounding 2 central microtubules), which is essential for their motility.

9 + 2 Arrangement

A specific structure found in most eukaryotic cilia and flagella, consisting of 9 outer doublet microtubules and 2 central singlet microtubules.

Basal Body

The structure that anchors cilia and flagella to the cell, composed of 9 triplet microtubules.

A Tubule

In the doublets for flagella/cilia, the ________ is the microtubule with dynein

Ciliary/flagellar beating

_______ is produced by the controlled sliding of doublet microtubules against each other