Overview of Cell Biology/Microtubule-Binding Proteins and Motors

Polymerization/Depolymerization Proteins
Tubulin binding proteins promote depolymerization and catastrophe (i.e. Stathmin, Op18).

Nucleating proteins nucleate MT assembly at the centrosome (i.e. γTURC).

End-binding proteins bind the (+) end of filaments and mediates chromosome/membrane interaction (i.e. CLIP-170).

Severing proteins oromote depolymerization at the (-) end of filaments (i.e. katanin).

Depolymerizing proteins promote depolymerization at the (+) end by inducing protofilament curling (i.e. Kinesin-13 family, KinI - the mechanism of KinI does not involve hydrolysis of the GTP cap).

General Properties
MAPs stabilize MTs by binding to sides and inhibiting disassembly. MAPs can also stabilize nuclei to promote assembly. MAPs function to organize MTs into bindles in various cellular structures and mediate MT interaction with other protens.

MAP Domains
- The MT binding domain binds several tublin dimers at once and help stabilize the polymer. - The projection domain interacts with MTs or other structures such as intermediate filaments

Map2 and Tau
MAP2 and Tau are responsible for organizing microtubules in neuronal axons and dendrites. Axon-like structures can be induced in cells that do not normally express axons if MAP2/Tau is introduced. The projection domain of MAP2 is larger, so the spacing between MTs is greater in MAP2 expressing cells.

Kinesins
Kinesis are MT-activated mechanochemical ATPases which undergo a crossbridge cycle similar to myosins.

N-type kinesins have motor domains at the N-terminus and move toward the (+) end of microtubules.

C-type kinesins have their motor domain at the C-termnus and move towards the (-) end of microtubules.

I-type kinesis have their motor domain in the internal of the protein sequence and do not move along microtubules. Instead, they bind to microtubule ends and promote depolymerization and peeling.

Kinesin-1 was the first kinesin discovered and is the best characterized in the kinesin family. It is composed of 2 heavy chains and 2 light chains. Kinesin-1 is an N-type kinesin that forms a coiled coil with another kinesin-1 tail.

Dyneins
Dyneins are huge (greater than 1,000,000 Da in molecular weight) motor molecules composed of 2-3 heavy chains complexed with a poorly characterized number of light chains.

Flagellar dynein is involved in cilliary and flagellar motility and the closely related cytoplasmic dynein is important for various other celullar functions.

The dynactin complex is a multisubunit protein complex that copurifies with dynein and is thought to mediate the attachment of dynein to vesicles and organelles.

Dynein motility is derived from ATP hydrolysis and moves towards the (-) end of microtubules.

Kinesin and Dynein Function
Transport from the cell body to the synapse is called anterograde transport and transport back to the cell body is called retrograde transport. Microtubules are oriented with their (+) ends towards the synapse; therefore, anterograde transport is dependent on kinesin, while retrograde transport is dependent on cytoplasmic dynein.

Intracellular movement an d positioning of organelles
Microtubles mediate the positioning of internal organelles such as the Golgi Apparatus and the Endoplasmic Reticulum. The Golgi is located near the centrosome. If the microtubules are disrupted by drugs, the Golgi becomes fragmented and the ER collapses. The ER reforms if the microtubules are repolarized.

Structure of Cilia and Flagella
The core concists of axonome in which 9 outer doublet microtubules surround a central pair of singlet microtubules. The doublets consist of A-microtubules (13 protofilaments) and B-microtubules (10 protofilaments). The outer doublets are connected by nexin. The other doublets are connected to the central microtubules via radial spokes. THe central microtubules are stabilized by an inner sheathDynein arms attached to the AMT drive ciliary beating.

The axonome arrizes from the basal body, which acts as a template for the formation of new flagellin. At the basal body, the axonome is an Amt-Bmt-Cmt triplet which becomes an Amt-Bmt doublet in the axonome.

Movement
Flagella moves in a sinusoidal and symmetric waveform. Cilia moves in an asymmertric, powerful stroke.

The sliding MT hypothesis postulates that flagellar and ciliary bending is powered by sliding of the outer doublet microtubules relative to one another toward the (-) end of MTs.

Experimental Evidence:

Axonomes with high salt concentration will be stripped of dynein. These isolated axonomes no longer beat. If dynein is re-introduced, beating is reconstituted