8.2 Microtubules and MAPs

As most of the microtubules in a cell or nucleated at the center zone what does this mean for the orientation of the microtubule?

The minus end of microtubules is generally found at the centrosome, where it is nucleated, and therefore the plus end of the microtubules will be found out at the edges of the cells

What does the protein in bacteria called filament temperature sensitive mutant Z(FtsZ) do?

similar to tubulin by playing an important role in the division of some bacteria and the replication of mitochondria and chloroplasts

What are microtubule associated proteins?

-MAPs affect stability, nucleation and turnover of microtubules by modifying the underlying ability of tubulin to assemble and disassemble -they also regulate the stability, distribution and organization of microtubules

How is microtubule assembly regulated?

-GTP binds to the beta subunit and facilitates the addition of the subunit onto the end of a growing microtubule. GTP that is bound to get hydrolyzed GDP -As long as the microtubule is growing rapidly GDP bound subunits will be buried in the microtubule by the addition of new GTP bound tubulin dimers and the newly added GTP bound subunits form what is known as the GTP cap -if the addition of GTP subunits slows down the subunits at the end of the microtubule hydrolyze the GTP and become GDP bound subunits and at that point the seven it's a weekend interactions with other subunits and the Protofilaments begin to splay out from each other. Once the Protofilaments splay out the microtubule can't regrow and it will depolymerize completely unless some other factors around to stabilize it -both polymerization and depolymerization can be occurring at the plus end of a microtubule which is a major difference between actin and microtubules

How do alpha and beta tumbling bind GTP?

-GTP that is attached to the alpha tubulin subunit is not hydrolyzed and is buried between the two proteins -GTP that is bound to the better subunit can be hydrolyzed and this hydrolysis plays a role in the assembly of microtubules

What organelles would have kinesins associated with them?

-Kinesins are a class of microtubule motor proteins which move along microtubules in the direction of the plus end of the microtubule -Kinesins stay attached to a microtubule for half of the ATPase cycle and have at least two motor subunits, this means that the protein barely falls off of the microtubule as one subunit is always attached. -The word processive is a term used to describe how long something is likely to continue once it gets started so we say that microtubule motors are highly processive but myosin is not -the plus end of microtubules are at the cell wall so organelles such as secretory vesicles would use kinesin motor proteins

What microtubule associated proteins have we discussed that would be expected to stabilize microtubule arrays? Which would destablize them?

-MAP2 is a relatively long microtubule associated protein that stabilizes microtubules and creates loose microtubule bundles. -Tau is a shorter MAP and makes tighter microtubule bundles. -The function of microtubule associated proteins is going to be regulated through phosphorylation and the phosphorylation of MAPs generally releases them from microtubules -So stable microtubules are generally associated with non-phosphorylated MAPs while the phosphorylation of MAPs tends to remove them from microtubules and results in microtubules being more dynamic and less stable. -MAP2 would create less stable microtubule arrays while Tau would make them more stable

Microtubules exhibit a behavior called "dynamic instability". Describe this behavior, its molecular basis, and its cellular significance.

-Microtubule turnover is usually characterized by Dynamic instability where the growing and shrinking of microtubules is present at the same time and allows the microtubule to "probe" the cell cytoplasm for targets like chromosomes -Dynamic instability is when a "catastrophe" occurs (sometimes by catastrophin) which causes the microtubule to shrink but it is then 'rescued' by a stabilizing factor. This cycle of assembly and disassembly can be repeated over and over again as long as free GTP-bound tubulin subunits are available -On a video Dynamic Instability would look like some microtubules in a population are growing while others are shrinking and is an essential component of the behavior of microtubules as it allows the microtubule to probe for targets.

What are microtubules?

-Polymers of smaller subunits just like actin filaments, however, the subunits that make up microtubules are dimers of two different proteins called alpha and beta tubulin that form rows called protofilaments -In protofilaments all the dimers point in one direction soon one end of the microtubule there are exposed alpha tubulin subunits and on the other end there are exposed beta tubulin subunits -13 protofilaments will form a complete microtubule

What is the function of proteins like EB1 in cells? Can you give an example?

-The TIP protein EB1 associates with GTP tubulin at growing microtubules and helps them attached to target sites -TIP proteins will mark the GTP cap at the end of a microtubule and helps to lead the growth of the microtubule and it will repeatedly attach and detach looking similar to treadmilling -An example would occur in a budding yeast cell where it will help to anchor one end of the mitotic spindle to the new cell

What is microtubule nucleation and where does it occur?

-The process in which several tubulin molecules interact to form a microtubule seed. Microtubules can assemble spontaneously but in cells most of the microtubules grow from some kind of nucleation site. -In non-dividing cells there is a single major site of microtubule nuclear nucleation called the centrosome it is also called the microtubule organizing center (MTOC) -In some cases the microtubules of the centriole, which are small barrel shaped arrays of microtubules embedded in the centrosome, play a role in the nucleation of microtubules directly such as in cilia and flagella -In most cases there are small ring-shaped protein complexes within the centrosome which form the microtubule nucleation site and are made up of gamma tubulin. So gamma tubulin ring complex generally forms the nucleation of microtubules

Microtubules are much less stable in motile cells than they are in non-motile cells cells. What might be the basis for these differences?

-movement of the cell basically involves disassembling in one place and reassembling it another so it makes sense that the microtubule arrays would have to be re-organized as the cell moves and so non-mobile cells would naturally be more stable -Motile cells would have microtubules that grow and shrink rapidly while non-motile cells have much more stability and long-lived microtubules

What organelles would have minus-end directed microtubule motor proteins associated with them?

Dynein motor proteins move organelles towards the minus end of a microtubule so organelles that need to be at the center of the Cell would use dynein motor proteins such as the Golgi apparatus

What are microtubules motor proteins?

Dyneins and kinesins which transport organelles, chromosomes and other cargoes along microtubules. They also support the movement of Cilia and flagella