Cytoskeleton 2.docx

Actin Actin is a 40kDa protein. Present in both muscle and non-muscle cells Each actin molecule has binding site for Mg2+ ion with either ATP or ADP bound. Assembly of G-actin into F-actin is accompanied by hydrolysis of ATP to ADP and Pi. Actin polymerisation Nucleation phase The subunits of G-a...

Actin Actin is a 40kDa protein. Present in both muscle and non-muscle cells Each actin molecule has binding site for Mg2+ ion with either ATP or ADP bound. Assembly of G-actin into F-actin is accompanied by hydrolysis of ATP to ADP and Pi. Actin polymerisation Nucleation phase The subunits of G-actin collide with each another Where monomers weak association with one another and eventually the nucleus forms. The subunits can attach to each other and release from one another. Forms oligomers. Critical concentration Cc= where the rate of subunit addition and loss are equal. The higher the concentration of actin subunits and more frequent collisions= higher chance of forming an oligomer. Elongation phase Actin subunits can add onto the filament very efficiently. Filament always grows on one side only- (key aspect). Becomes F-actin and all subunits face the same way. ATP becomes hydrolysed and becomes ADP and pi for the energy requirement. Steady phase Filament remains the same size. Subunits come on and off. Dynamic process, filament stays the same size High Cc= more subunits come together Low Cc= the subunits assemble and disassemble Tubulin Made up of alpha and beta tubulin molecules. Forms heterodimer Polymers of alpha and beta tubulin are arranged in tubules with 13 protofilaments. Dimers stick together with the use of nucleotide GTP which provides energy. The GTP is considered an integral part of the tubulin structure due to bring bound to alpha tubulin and its never hydrolysed or exchanged. Steps of polymerisation for microtubules Tubulin dimers forms when alpha and beta tubulin subunits adhere together. The dimers attach to form oligomers. (nucleation stage) Forms protofilament (has 13 of them) Forms sheets of protofilament Forms a elongated hollow tube