Microfilaments and microtubules

Microfilaments and Microtubules. There are two important classes of fibers found in the cytoplasm of many plant and animal ceUs that are characterized by nematic-like organization. These are the microfilaments and microtubules which play a central role in the determination of ceU shape, either as the dynamic element in the contractile mechanism or as the basic cytoskeleton. Microfilaments are proteinaceous bundles having diameters of 6—10 nm that are chemically similar to actin and myosin muscle ceUs. Microtubules also are formed from globular elements, but consist of hoUow tubes that are about 30 nm in diameter, uniform, and highly rigid. Both of these assemblages are found beneath the ceU membrane in a linear organization that is similar to the nematic Hquid crystal stmcture. 

The cytoskeleton also contains different accessory proteins, which, in accordance with their affinities and functions, are designated as microtubule-associated proteins (MAPs), actin-binding proteins (ABPs), intermediate-filament-associated proteins (IFAPs), and myosin-binding proteins. This chapter is focused on those parts of the cytoskeleton that are composed of microfilaments and microtubules and their associated proteins. The subject of intermediate filaments is dealt with in detail in Volume 2. 

In this chapter we describe the distribution, assembly, and interaction of microfilaments and microtubules and their functional roles in cell movement and in the maintenance of the spatial organization of the cytoplasm. Also, the relative roles... 

Microfilament and microtubule dynamics underlie growth cone motility and function 132... 

Although intermediate filaments are not universally associated with the cytoskeleton, neutrophils possess intermediate filaments of the vimentin type. Vimentin is a rod-shaped molecule of relative molecular mass 57 kDa that readily polymerises under physiological conditions to produce stable filaments 10-12 nm in diameter. Intermediate filaments are more robust than microfilaments and microtubules, and in neutrophils they form an open network of single filaments in the perinuclear space. 

Both actin microfilaments and microtubules (see below) are involved in uptake as well as in the pathway to the lysosomes and the recycling pathway. Referring to clathrin-mediated uptake, Durrbach et al. showed actin participation in two steps of endocytosis the initial uptake mechanism and the delivery to lysosomes (116). 

Free protein monomers of intermediate filaments rarely occur in the cytoplasm, in contrast to microfilaments and microtubules. Their polymerization leads to stable polymers that have no polarity. 

In addition, several annexins have been shown to interact directly or indirectly with actin microfilaments and microtubules, thus annexins have the potential to form a structural link between the membrane and the underlying cortical cytoskeleton (reviewed in Gerke et al. 2005 Hayes et al., 2004). As well as lateral inter-annexin interactions (either homotypic or heterotypic with other types of annexin)... 

Together with actin microfilaments and microtubules, keratin filaments make up the cytoskeleton of vertebrate epithelial cells. Keratins belong to a family of intermediate filament proteins that form a-helical coiled-coil dimers that associate laterally and end to end to form 10 nm diameter filaments. Keratin and actin filaments and microtubules form an integrated cytoskeleton that preserves the shape and structural integrity of the ker-atinocyte as well as serves to transmit mechanical loads. Keratins account for about 30% of the total protein in basal cells. 

Understand the composition, structure, and function of microfilaments and microtubules. 

Cytoskeleton. Many cellular activities, such as motility, endocytosis, exocytosis, and cell division, rely on microfilaments and microtubules. A number of alkaloids have been detected which can interfere with the assembly or disassembly of microtubules (Table IV), namely, vincristine, vinblastine, colchicine, maytansine, maytansinine, and taxol. 

Dendrites are the (filamentous) terminal portions of neuron that bind neurotransrrritter chemicals migrating across the synaptic gaps separating neurons. Depending on the type and function of a particular neuron, neurotransmitters may cause or inhibit the transmission of neural impulses. The cell body contains the cell nucleus and a concentration of cellular organelles. The cell body is the site of the normal metabolic reactions that allow the cell to remain viable. Neurotransmitters synthesized within the cell body are transported to the axon terminus by microfilaments and microtubules. 

Although Intermediate filaments are much more stable than microfilaments and microtubules, they readily exchange subunits from a soluble pool. 

The mechanical properties of living cells are significantly determined by the cytoskeleton. There are three major classes of protein filaments that belong to the cytoskeleton and have individual functions microfilaments, intermediate filaments, and microtubules [39]. Whereas microfilaments and microtubules are highly dynamic structiues that can form rapidly by polymerization of actin or tubuHn monomers, respectively, the intermediate filaments are stationary structures that are important for the basal mechanical stabihty of the cell but not for dynamic changes. Since the microfilaments are composed of filamentous actin they are also referred to as actin filaments [39]. Both expressions can be used synonymous. 

The intermediate filament protein (IF protein) is a major component of animal cells along with microfilaments and microtubules. Compared to microtubules and microfilaments, a single intermediate filament tends to be rather flexible, extensible, and tough [44]. 

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