BioS222_reading notes

Page 1

PAGE 577-590______________________________________

Microtubules = in eucaryotic cells, long relatively stiff can rapidly assemble and disassemble in different locations

Microtubules grow from centrosomes that are located in the center of the cell.

entering mitosis cytoplasmic microtubules disassemble and reassemble in mitotic spindle

Microtubules can also be cilia or flagella

motor proteins propel organelles along the cytoskeletal tracks

polarity of ends in microtubules is crucial to their function, especially for assembly and function

Centrosome, located near the nucleus, organizes array of microtubules that radiate outward from it and go through the cytoplasm

centrioles , pair of them, function is a mystery, but plant cells lack them

cells can control where microtubules form

dynamic instability, stems from tubulin molecules hydrolyzing GTP

high rate of polymerization causes tubulin molecules add to the end faster than GTP can be hydrolyzed, end of growing microtubule is then composed of primarily go GTP subunits forming the GTP cap.

Instability of microtubules allow them to undergo rapid remodeling, crucial for microtubule function

a microtubule can be prevented from growing if its plus end is some how attached to another molecule or cell structure, preventing depolymerzation

drugs that prevent polymerization or depolymerzation of tubulin can have a rapid and profound effect on the cytoskeleton and behavior of the cell

when cells enter mitosis become much more dynamic, more than usual

when microtubule differentiates into a specialized cell type , dynamic instability becomes suppressed.

Most all animal cells are polarized, so one end of the cell is structurally different than the other end

Microtubules often work in conjunction with other proteins to help carry out different functions.

cytoplasm is in continuous motion

Saltatory movement = jerky movement, move for short period of time and then stops for a short period of time. Benefits are much more sustained movement and directional than the continual movement (small Brownian movement, caused by random thermal motion).

Motor proteins = have ability to attach to other cell components and can transport cargo along filaments, they use energy derived from repeated cycles of ATP hydrolysis. can travel steady along actin filament or microtubule in a single direction.

motor proteins that move along cytoplasmic microtubules, have two types.

1) Kinesins = generally move towards positive end of microtubule(away from centrosome, outward from cell body).

2) dyneins = move towards minus end (toward centrosome).

The tail of a motor protein general determines the type of cargo that the motor protein will transfer

The globular heads of kinesin and dynein are enzymes with ATP-hydrolyzing activity.

Components of cells are able to be positioned where they need to be due to the microtubules

Colchicine destroys these motor proteins causing components like the endoplasmic reticulum and Golgi apparatus to lose its specific posing in the cell

stable microtubules are employed as still supporters to make things like cilia and flagella

Cilium is made up of: a core of stable microtubules,arranged in a bundle that grow from a basal body in the cytoplasm(basal body is an organizing center for the cilium.)

function of cilia is to move fluid over the surface of the cell or propel single cells through a fluid.

closely related to the cytoplasmic dynein, Motor protein ciliary dynein generates bending motion of the core. Works by the cilary dynein attaches its tail to one microtubule while its head interact with an adjacent microtubule to generate a sliding force between the two filaments

page 571-572________________________________________________________________________________________

Cytoskeleton = gives eucaryotic cells the ability to adopt different shapes, organize components of cell, enter act mechanically with the environment, carry out coordinates movements

Cytoskeleton is a highly dynamic structure.

cytoskeleton controls location of organelles that conduct specialized functions; large scale movement(responsible for muscle contraction, crawling of cells)

CHECK OUT CYTOSKELETON MINDMAP

intermediate filaments have great tensile strength

Main function of intermediate filaments is to enable cells to withstand the mechanical stress that occurs when the cell is being stretched.

intermediate because they can be found in-between thin actin filament and thick myosin filament

most durable type of filament found in cytoplasm

often anchored in plasma membrane at cell-cell junctions(desosomes)

PAGE 605-606__________________________________________________________________

cytoplasm of of cell is organized by cytoskeleton intermediate filaments, microtubules, and actin filaments

Intermediate filament are stable, rope like polymers of fibrous proteins that give cells mechanical strength

Microtubules are stiff hollow tubes formed by polymerization of tubulin dimer subunits (plus and minus ends)

Many Microtubules in a cell are in a liable, dynamic state in which they alternate between a growing state ands a shrinking state. Transitions are known as dynamic instability and are controlled by hydrolysis of GTP bound to tubulin dimers.

each tubulin dimer as a tightly bounded GTP molecule that is hydrolyzed to GDP after the tubulin has assembled into a microtubule.

GTP hydrolysis reduces affinity of the subunit for its neighbors and decreases the stability of the polymer, causing it to disassemble.

Microtubules can be stabilized by proteins that capture the plus end - a process that influences the position of microtubule arrays in a cell.

Cells contain microtubule associated proteins that stabilize molecules, bind them to other cell components, and harness them for specific functions

Kinesin and dyneine are motor proteins that use the energy of ATP hydrolysis to move unidirectionally along microtubules.

--------------------------->and----------->

carry specific membrane vesicles and other cargoes and in this way help to maintain the spatial organization of the cytoplasm

Eucaryotic cilia and flagella contain abundle of stable microtubules. Their beating is caused by bending of the micrtubule,driven by a motoprotein calle ciliary dynein

Actin filaments are helical polymers of actin molecules. They are more flexible than microtubules and are generally found in bundles or networks

Actin filaments are polarized structures with a fast and slow growing end, and their assembly and disassembly are controlled by the hydrolysis of ATP tightly bound to each actin monomer

Starting on page 590-605_____________________________________________________________________________________________________________________________________

Acitn filaments are important in cell movements especially on the surface. Actin act as movement for cell

actin filaments are unstable, but when binded with other proteins they can form stable structure(similar to microtubules)

know as actin-binding proteins, actin filaments tend to interact with a large number

Actin filaments can form stiff relatively permanent structures(microvilli), or small contractile bundles in the cytoplasm ant (can act like contracting muscles of the cell),contractile ring in t when cell divides

each actin filament is a twisted chain of identical globular actin molecules , point in the same direction, very similar to microtubules, forming a plus and a mini end.

Actin filaments are thinner and more flexible and usually shorter than microtubules.

Actin filaments are more abundant than microtubules.

actin filaments can grow by addition of actin monomers from either end, faster growth at positive end.

Actin filaments unstable(like microtubules), can disassemble from both ends

hydrolysis of ATP to GDP in an actin filaments reduces the strength of the binding between monomers and decrease the stability of the polymer

Key term is NUCLEOTIDE HYDROLYSIS : promotes depolymerization helping the cell disassemble filaments after they have formed.

Cytochalasins : prevent actin polymerization

Many proteins can bind to actin and modify its properties

about 5% of protein in animal cell is actin , half is assembles into actin filament, other half remains as actin monomers in cytosol of cell.

Cells contain thymosin and profilin (proteins) that bind to monomers in cytosol , preventing them from adding to the ends of actin filaments, therefore preventing actin monomers to polymerize into actin filaments

Thymosin and profiling play crucial role in regulating actin polymerization

Proteins like forming or actin-related proteins(ARPs) control actin assembly

vs

Cell Cortex is the layer of cytoplasm just beneath the plasma membrane

Filament severing proteins (gelsolin) work by fragmenting actin filaments into shorter lengths and

L_15/16

L_17/18