Cytoskeleton introduction Cytoskeleton proteins > Types of cytoskeleton Filaments are long chains of proteins, such as those found in hair, muscle, or in flagella. They are often bundled together for strength and rigidity. · Actin Filaments · Intermediate filaments · Microtubules > Cytoskeleton: an intricate network of protein filaments that extends throughout the cell. > Functions of the cytoskeleton . Controlling the location of the organelles · Providing machinery for transport between organelles · Segregating chromosomes into two daughter cells at cell division and pinching apart those two new cells Cytoskeleton proteins- actin filaments 1 Filaments · Actin Filaments The thinnest filaments of the cytoskeleton, a structure found in the cytoplasm of cells. These linear polymers of actin subunits are flexible and relatively strong, resisting buckling due to compressive forces and filament fracture by tensile forces on the cells. Each actin molecule has binding sites for Mg2+ion with either ATP or ADP bound. Actin exists in two principal forms, i.e. globular monomeric form (G) and filamentous polymeric form (F) (F form is also the functional/active form) > Cellular functions . Determines the shape of the cell's surface · Whole-cell locomotion · Cell division > Physiological functions (in association with Actin Binding proteins) · Body muscle contraction · Heart function > Dysfunctions · Congenital myopathies Actin-accumulation myopathy (severe muscle weakness (myopathy) and poor muscle tone (hypotonia) throughout the body) Cytoskeleton proteins- microtubules > Filaments · Microtubules A component of the cytoskeleton found throughout the cytoplasm. These tubular polymers of tubulin can grow as long as 50 micrometres and are highly dynamic. The outer diameter of a microtubule is about 24 nm while the inner diameter is about 12 nm. Structure of microtubules · There are two types of tubulin molecules, a tubulin and ß tubulin, which form a heterodimer. · Both a monomer and ß monomer can bind to GTP. GTP bound to the a-tubulin is trapped at the dimer interface and is never hydrolysed or exchanged, while the B-tubulin can bind to either GTP or GDP.
Regulation of microtubule- microtubule associated proteins (MAPs) MAP Functions Stabilizing MAPs MAP2: binds along the microtubule lattice and stabilizes microtubules Tau: possesses a microtubule crosslinking domain Plectin: links to intermediate filaments Destabilizing MAPs E.g. Stathmin: binds tubulin dimers & prevents assembly Katanin: severs microtubules Plus end track protein XMAP215: stabilizes plus ends and accelerates assembly Plus end tracking proteins (+TIPs): remain associated with growing plus ends and can link plus ends to other structures Motor Proteins E.g. Kinesins and Dyneins, transport cargo and perform a variety of other cellular functions. Cytoskeleton proteins - microtubules > Cellular functions Localizing and transporting organelles Cell division > Physiological functions Neuron migration Motile cilia: clearing dirt, dust, micro-organisms and mucus, to prevent disease > Dysfunctions Alzheimer's disease (Hyperphosphorylation of Tau) Cytoskeleton proteins- intermediate filaments 1. Filaments · Intermediate filaments With an average diameter of 10 nanometres, which is between that of 7 nm actin (microfilaments), and that of 25 nm microtubules, and they were initially designated 'intermediate' because