Cytoskeleton

Cytoskeleton and Associated Proteins

Microfilaments

Microfilaments are formed from actin polymers Actin is abundent in cells: 5% of protein composition Not random Structure Found as single strands, parallel bundles, and networks Microfilaments can be arranged in stress fibers to give cells high tensile strength Just beneath the cell membrane is the cortex, which is dense with microfilaments Found in adhesion junctions Focal contacts actin connection to ECM components Actin network can change its consistency: Gel --> Sol --> Gel --> Sol --> Locomotion Amoeboid locomotion Viscosity of the cytoplasm can be liquid --> gel this depends on the degree of cross linking by certain actin binding proteins (ABP's) regulation of ABP calcium dependent IP3 / DAG system CAMP calcium channel / calmodulin system Actin monomers --> Actin fillaments needs a seed crystal to form nucleation site in order for monomers to form polymers form pointed end and barbed end: actin monomers most easily add at the barbed end and dissociate at the pointed end [Critical] is gained where addition = dissociation which results in a phenomenon called "treadmilling" How does pseudopod movement occur Pulling or pushing of cytoplasm by actin Later it was thought that it was actin and myosin like in muscle tissue New hypothesis as seen in phagocytes binding of antibody depolymerizes the actin in the cell increase of solute causes cytosol water to flow in concentration gradient cell is desensitized to stimulus polymerization of actin occurs Two proteins are involved Gelsolin: breaks up actin and binds to barbed end; calcium dependent Profilin: binds to actin monomers and ensures that polymerization does not occur Misc. bound receptor signals inositol to release calcium and gelsolin breaks down actin actin reassembly stabilizes pseudopod: associated with cell membrane and breaks apart complexes gelsolin / actin profilin / monomer

Microtubule Structure, Mobility, and MAP's Tubulin Alpha tubulin subunit: contains GTP Beta tubulin subunit: contains GDP dimer polymerizes Assembly and disassembly either in steady growth or rapid disassembly: called "Dynamic Instability" Control factors [GTP] [Heterodimer] [Calcium]: an increase inhibits polymerization Covalent modification: usually stabilizes phosphorylation acetylation detyrosination: occurs at the carboxy terminus Inherent directionality: heterodimers add and subtract at both ends, but preferentially add at the positive GTP end GTP end forms the cap: if GTP cap is allowed to hydrolize, then GDP cap favors rapid depolymerization If heterodimer concentration in the cell is high then polymerization occurs When [heterodimer] falls low GTP cap is hydrolized and lost: rapid depolymerization occurs negative end anchoring at microtubule organizing center (MOTC's): basal bodies, centriole, and centrosome

Microtubules and Cell Motility Centriole 9 + 2 is found in flagella (negative end), and is attached to basal bodies, but In cell division they are attached to MOTC and centrioles Microtubules: associated with cilia / flagella (eukaryotic) cilia: whiplike motion flagella: wavelike motion Flagellum is surrounded by the plasma membrane and has a 9 + 2 arrangement: the cross section is called axoneme Inner doublet is made of two complete tubules surrounded by innersheath all doublets are connected with nexin radial spoke connects 9 --> 2 in the center Dynein provide the "power stroke" for movement fillaments try to slide like in muscles but the negative end is anchored to the cell and instead a bending motion occurs Microtubules role in cell transport: "tracks" for directional transport kinesin --> MAP kinesin moves cellular molecules dyenin also acts here but it carries vesicles in the opposite direction as kinesin

Intermediate Fillaments and Associated Proteinss (IFAP's) A diagnostic tool used in cancer diagnosis because cells retain their characteristic IF type More stable and less soluable than mictofillaments and microtubules IFAP's are cross linkers and caps Structure Monomer is rodlike and fiberous: four alpha helices with a globular region on both ends alpha helix regions are conserved and globular ends are diverse Two monomers form to make a dimer Form antiparallel tetramer, and line up lengthwise to form a protofillament Protofibril --> intermediate fillament: overlapping of this is critical to IF strength Five classes of intermediate fillaments due to the diversity of the globular regions I, II: Keratin epithelial tissues line and cover the body structures from desmosomes and hemidesmosomes III: Vimentin connective tissue cells blood, bone, and cartilage III: Desmin muscle cells smooth, cardiac, and skeletal form bracing elements and Z lines IV: Neurofillaments nerve cells especially in axons and confer strength III: Glial fillaments nerve cells in CNS V: Nuclear lamins outside and inside the nuclear envelope Phosphorylation: confers stability disassembly Dephosphorylation: confers stability re-assembly