Cell Bio Chapter 9 The Cytoskeleton and Cell Motility

Cytoskeleton

Network of filamentous structures in cells.

Cytoskeleton is composed of

microtubules, microfilaments, intermediate filaments

Cytoskeleton Function

It serves as a scaffold providing structural support, organizes organelles within the cell, directs the movement of materials and organelles, Generates force to move cells, and is an essential component of the cell's division machinery.

Microtubules

Long, hollow, relatively rigid, unbranched tubes that universally exist in the cytoplasm of eukaryotic cells

Microtubules function

Provide mechanical support, organize organelles, and serve as tract for transport

Protofilament

Linear array of tubulin subunits in microtubules.

Βeta-tubulin end of Protofilament

plus end

Αlpha-tubulin end of Protofilament

minus end

number of protofilaments in microtubule

13

MAPs

Proteins stabilizing and connecting microtubules.

Vesicle transport

Microtubules facilitate movement of vesicles.

Motor proteins

the proteins required to interact with cytoskeleton to provide strength for the movement

Kinesin

Plus end-directed microtubular motor protein moving along microtubules.

Dynein

Minus end-directed microtubular motor protein moving along microtubules.

Dimeric building blocks

α- and ß-tubulin forming protofilament.

Microtubule-Organizing Centers (MTOCs)

specialized structures for the nucleation (assembly) of microtubules

Centrosomes

Best studied microtubule-organizing centers in animals.

Centrosomes function

Responsible for initiation and organization of microtubules in animal cells

Pericentriolar Material (PCM).

Initiates assemble of microtubules

Centrioles

Barrel-shaped structures within centrosomes.

Microtubule Polarity

Minus end is associated with the centrosome, Plus (or growing) end is at the opposite tip.

γ-tubulin

required for microtubule nucleation and located in the centrosomes.

γ-tubulin Ring Complex

Structure that aids microtubule nucleation.

αβ-tubulin Dimers

Building blocks that bind to γ-tubulin.

Basal Body

MTOC responsible for the initiation and organization of microtubules for outer microtubules in cilia and flagella.

Plant Cell Microtubules

Organized around the nucleus, lacking MTOCs.

Microtubule Growth

occurs by the addition of subunits at the plus end of the polymer away from the centrosome

Functions of MTOCs

Initiates microtubule nucleation, Control the number of microtubules, polarity of microtubules, number of protofilaments that make up the wall of microtubules, and time and location of microtubule assembly.

Dynamic Properties of Microtubules

Microtubules can assemble or disassemble as needed.

Intermediate Filaments (IF)

Strong, flexible rope-like filaments only in animal cells with no polarity.

Composition of IF

Made of heterogeneous proteins, ~70 identified.

Classes of IF

Five major classes including lamins.

IF Assembly

Monomers form dimers, then tetramers.

Tetramer

Building block for IF made of two dimers aligned antiparallel, no polarity.

IF Filament Formation

Made from eight tetramers

Keratin Filaments

Primary structural protein for epithelial cells by anchoring to the nuclear envelope and connecting to the cell's outer edge through hemidesmosomes and desmosomes.

IF Function

serves as a scaffold for organizing and Support structure for cell and for absorbing the mechanical stress applied by the extracellular environment

Polarity of IF

Dimers have C-termini and N-termini ends.

Cross-bridges in IF

Interconnect IFs with other cytoskeletal filaments.

Actin filament

Also known as microfilament or F-actin, thinnest cytoskeleton structure with polarity.

Actin protein

Single component forming actin filaments.

Actin filament functions

Responsible for motility of cell and intracellular parts (White blood cell patrol and Movement of cilia lining the respiratory tract)

ATP-bound actin

Building block for microfilament assembly.

Nucleation of microfilament

Process requiring preformed filament as seed.

Barbed End/Plus End

Fast-growing end of microfilament, high ATP affinity.

Pointed End/Minus End

Slow-growing end of microfilament, low ATP affinity.

Critical concentration

Minimum ATP-actin concentration for filament elongation.

Steady state

where length of microfilament and free ATP-actin remain constant.

Myosin

The molecular motor of actin, moving along microfilaments tracks

Conventional (Type II) myosin

The primary motor for muscle contraction moves toward the barbed end (+). Moves unidirectionally and has Actin filament moved to the minus end.

Unconventional (Type I) myosin

Transport vesicles, moves differently than conventional myosin.

Function of Myosin

Muscle contraction, Required for splitting a cell(cytokinesis), Generation tension at focal adhesion and Adherens junctions, Cell migration, Intracellular Transportation