Regulation of Cytoskeletal Filaments Pages 992-1010

Most cytoskeletal regulation is performed by accessory proteins that bind to either the filaments of their free subunits. Microtubule-organizing center (MTOC) – a specific intracellular location where microtubule nucleation occurs

Nucleation of Microtubules by gamma-tubulin - end + end

The Centrosome, a MTOC 50 gamma-TuRC

Cross-section of a Centrosome The centrioles organize the centrosome matrix ensuring its duplication during each cell cycle Centrioles are composed of a short cylinder of modified microtubules and a large number of accessory proteins Neither fungi nor most plants have centrioles Centriole

Center-Seeking Behavior of a Centrosome

Reorganization of Microtubules

Nucleation by the ARP Complex ARP – actin-related protein, each ARP is about 45% identical to actin

Structures of Actin, Arp2, and Arp3

Actin Web Formation

Binding of Profilin and Thymosin About 50% of actin in nonmuscle cells is in filaments and 50% as soluble monomers

Profin bound to Actin Monomer - Profilin binds to the opposite side of the ATP-binding site, blocking the side of the monomer that would associate with the filament minus end, allowing this complex to add onto a free plus end +

Effects of Thymosin and Protilin on Actin Polymerization

Effects of Stathmin on Microtubules Stathmin’s binding to tubulin is inhibited by the phosphorylation of stathmin

Organization of Microtubule Bundles MAP – Microtubule-associated protein MAPs have at least 1 domain that binds to the microtubule and another that projects outward

Localization of MAPS in a Neuron -MAP2 protein stained orange in the cell body and dendrites -tau stained green in the axon

Actin Filament Twisting Induced by Cofilin Cofilin – is a small protein that binds actin in a 1:1 ratio and destabilizes actin filaments Tropomyosin – an elongated protein that bind simultaneously to 7 actin monomers and stabilizes actin filaments

Filament Capping Changes Filament Dynamics CapZ – Capping protein Capping is regulated by intracellular signals, PIP2 (Phosphatidylinositol 4,5-bisphosphate) uncaps + ends

Proteins Binding Microtubule Ends Proteins that bind to the ends of microtubules can control microtubule positioning

Cross-linking Cyoskeletal Elements Red – Microtubules Blue – Intermediate Filaments Green – Cross-linking protein, Plectin Plectin also links IF to actin filaments and microtubules Filaggrin bundles keratin filaments in the epidermis of the skin to give it its toughness

Various Actin Arrays

Actin Cross-linking Proteins

Formation of 2 Types of Bundles

A Micorvillus

Filamin Cross-links Actin into a 3-Dimensional Network

Loss of Filamin Causes Abnormal Cell Motility Actin formed by filamin is required for cells to extend the thin sheet-like membrane projections call lamellipodia

Filament Severing Changes Filament Dynamics

Microtubule Severing Red - Microtubules

Actin Filament Severing by Gelsolin -Activated by high levels of cytoplasmic calcium -No energy needed -Gelsolin is removed by PIP2 Severing of microtubules by Katanin -made up of 2 subunits, one for severing and the other for targeting it -the process requires ATP

Platelet Activation

Platelet Activation

Focal Contacts in Fibroblasts Focal contacts –highly specialized type of attachment between actin filaments and the extracellular matrix

Effects of Extracellular Signals