Assembly of Actin Monomers•Actin filaments are composed of globular G-actin monomersthat assemble in the presence of K+ and Mg2+ into a double-stranded helix of filamentous F-actin•Filamentous actin (F-actin) is a linear structure consisting of two protofilaments that twist along their length•The control and regulation of the polymerization process depends on the local concentration of G-actin and the interaction of actin-binding proteins (ABPs), which can prevent or enhance polymerization.22
23Cytoskeletal filaments are dynamic: Constantly undergoing assembly and disassemblyTREADMILLINGDYNAMIC INSTABILITYPredominant with F-actinPredominant with MicrotubulesBoth are associated with nucleoside triphosphate hydrolysis.
24Dynamic Instability of Microtubules is a Flip-flop Between GTP-cap and No GTP-capAlso see Panel 16-2
Motor Proteins•Motor proteins called kinesinsand dyneins transport cargo along microtubules•Motor proteins are powered by ATP hydrolysis.•Intermediate filaments are not used as tracks by motor proteins.25
Myosin Family of Mechanochemical ProteinsMyosins have the ability to convert the energy released by ATP hydrolysis into mechanical work.
Sliding of Myosin Along Actin Filaments Causes Muscles to Contract