CytoskeletonMark Wiser

Three Major Filamentous Systems of the Cytoskeleton

Cytoskeletal ElementDescription (diameter)Protein Compositionmicrofilamentsthin filaments (6-7 nm) actin + associated proteinsmicrotubulestubular structures (25 nm) tubulin + associated proteinsintermediate filamentsrope-like fibers (~10 nm) IF proteins

Actin and Microfilamentshighly conserved globular protein (G-actin)polymerizes into thin filaments (F-actin)exhibits polarity (+ and ends)(barbed and pointed ends)filaments stabilized by other proteins

Actin Polymerizationrequires nucleation (activation)elongation primarily at + end (= barbed end)ATP/ADPG-actin has bound ATPADP-actin more likely to dissociateactin-binding proteinsmonomer sequestration (eg., profilin)capping proteins (eg., gelsolin)cellular regulationrho family (ras-like G-proteins)trimeric G-protiensdrugscytochalasins (prevent assembly)phalloidin (prevent disassembly)

Examples of Actin-Binding Proteins

Membrane Interactionssubmembrane cytoskeleton (eg., RBC)focal adhesions (ECM interactions)adherens junctions (cell-cell interactions)

Focal Adhesions

Myosin and Force Generationlarge family of proteins (16)actin-activated ATPaseconverts chemical energy into mechanical energy

Ameboid Movementreorganization of cytoskeleton in pseudopodiaforce for protrusion?force for traction?

MicrotubulesMajor Roles:

Mechanical/Cell ShapeMitotic SpindleCilia/FlagellaTubulin:

highly conservedheterodimer (a, b)in vivo assembly from MTOC

Microtubule Organizing Centerscentrioles aka basal bodies of flagellabarrel-shaped (triplets of mT)centrosomesamorphous matrix ( centrioles)organizes cytoplasmic mT arrayforms spindle during cell divisionspindle pole bodieslocated on nuclear membraneforms mitotic spindle in many lower eukaryotes

Flagellar Movement

Motor ProteinsmF associated

myosin

mT associated

dyneincilia/flagellacytoplasmickinesin

Kinesinslarge superfamily of proteinsdefined by 'motor' domaintail regions highly divergent (function)chemomechanical cyclesimilarities to myosintwo motors needed

Kinesins and Mitosisreorganization of mT during mitosisdisassembly of cytoplasmic mTassembly of spindle apparatusduplication of centrosome to form spindle apparatusat least 4 kinesins implicatedseparation of polesmigration of chromosomes

possible kinesin roles:

stabilize metaphase planesliding mT to drive poles apartpull asters toward membranekinetochore: chromosome movement

Intermediate Filamentsrope-like fibers extending from nucleus peripheryextremely stableresistant to detergent extractiononly in metazoasubunits are part of large multigene familyrelated to nuclear laminstissue specific expression

Intermediate Filament Proteins

TypeExamplesCell TypeSize (kDa)Iacidic keratinsepithelial 40-60IIbasic-neutral keratinsepithelial50-70IIIvimentinmesenchymal53desminmuscle52GFAPglial cells51IVneurofilamentneurons57-150Vlamins (A, B, C)nuclear lamina60-70

IF Proposed Structure

IF Functionmechanical supportabundant in cells and structures under mechanical stress (eg, epithelia, hair, nails, muscle, etc)genetic defects linked to keratin: epidemolysis Bullosa simplex, epidermolytic hyperkeratosislinks to membranes and other cytoskeletal elementsspecialized function in differentiated cellstissue specific expressionmarkers for tumor diagnosis