25.6: Cytoskeletal Accessory Proteins
The cytoskeleton is an essential cell component that plays several structural and functional roles. However, the filaments that make up the cytoskeleton cannot function independently and depend on the accessory or ancillary proteins to effectively carry out their function. Accessory proteins associate with cytoskeletal filaments and their monomers, aiding filament formation and function. They also help in the cross-communication among cytoskeletal filaments. Cytoskeletal accessory proteins are found in both prokaryotes and eukaryotes. However, those found in bacteria differ significantly between species. The evolution of these proteins from prokaryotes to eukaryotes is currently unknown.
Accessory proteins can mediate interactions between the two polymers within the cytoskeleton when associated with microtubules and microfilaments. Accessory proteins known as microtubule-associated proteins or MAPs increase the viscosity of actin-microtubule mixtures. These proteins are also known to regulate motor proteins, which associate with microfilaments and microtubules to facilitate the intracellular transport of cargo such as organelles, vesicles, and various macromolecules. Proteins such as kinesin and dyneins-dynactin complex regulate the motion of motor proteins through mechanisms such as regulation of ATPase rate, force generation, control of affinity towards the cargo or filament, and geometric arrangement of the motor proteins on the cargo.
Proteins that help intermediate filaments associate with other cytoskeletal filaments are intermediate filament accessory proteins. For example, the protein plectin forms 2–3 nm projections along intermediate filaments made of the protein vimentin. These projections help to cross-link these intermediate filaments to microtubules. Like other proteins, post-translational modifications also regulate the activity of accessory proteins. For example, MAP-IB, an accessory protein responsible for interactions between microfilaments and microtubules, cannot bind to microfilaments when they are phosphorylated. However, dephosphorylation of this protein can bind to microfilaments and promote interactions between microtubules and microfilaments.
