Microtubule Associated Proteins MAPs

The full range of MT functions is achieved by the actions of various proteins which bind in precise fashions at specific tubulin dimers in MT lattices. These microtubule associated proteins (MAPs) include electromechanical enzymes which generate force and movement, communicative crossbridges to other cytoskeletal filaments and organelles, MAPs which enhance MT assembly (Figure 5.12) and a class of MAPs within neurons whose functions are not understood.

In many cases, the attachment patterns of MAPs to MT lattice walls have a precise geometrical configuration which appears related to the function of the MAP-MT complex (Figures 5.13 thru 5.16). For example Allen (1979, 1985) and colleagues showed that smooth passage of vesicles in axoplasmic transport requires contractile MAPs spaced between 15 and 25 nanometers apart along MT lengths. Many other MAP binding patterns are spirals which wind around MT cylinders in super-helices, and several theories suggest how these sites can be determined. These include linear sequencing of varying tubulin isozymes along protofilaments (Kim, 1986), crystal symmetry describing each MAP terrain (Figure 5.17, Koruga, 1986) or by the lattice steps necessary to get from one MAP site to its nearest neighbor MAP. For example, electron micrographs of MAP-MT binding by Burns (1978) clearly show MAP binding which may be described as "over 3, up 1." That is, if the tubulin subunit to which the MAP is bound is the starting point, moving 3 monomers along the leftward row helix, and then up 1 monomer along the protofilament will indicate the next attachment site. A number of geometrical super-helical patterns have been observed which match other "chess-like" movement rules. These are shown in Figures 5.12-5.15. In other instances such as brain MAPs, attachment patterns are irregular and heterogeneous, and thus are capable of representing information.

Figure 5.13: Patterns of MAP attachment to microtubules observed by electron microscopy (Burns, 1978; Kim et al., 1986; and others).

Rod shaped bridges which occur between MT are of at least two sorts. Motion producing protein "arms" which consume ATP hydrolysis energy to generate force are analogous to myosin bridges of skeletal muscle. Moving arms attached to MT are made of proteins called kinesin and dynein. Dynein arms on MT which contract in organized sequences to produce collective movements were first described and characterized in cilia and flagella by Ian Gibbons (1968). The arms reside at periodic intervals along the outer MT within cilia and flagella. They possess "ATPase activity," that is they .contract conformationally due to hydrolysis of ATP. Dynein arms contract in waves and mediate sliding between the adjacent outer tubules to drive ciliary and flagellar motility. Dynein arms attached to MT in neuronal axons act collectively to pass material in "bucket brigade" axoplasmic transport. These and other MT "bending sidearms" will be discussed in Section 5.5.2.

Other MAPs are bridges between parallel MT, filaments, organelles and the "microtrabecular lattice" and appear to integrate microtubules with the rest of the cytoplasm (Figure 5.6). MAPs from mammalian brain neurons have been characterized into three main groups. These are high molecular weight (greater than 100 kilodalton) MAP 1 and MAP 2, and a number of closely related 56-62 kilodalton proteins designated as "tau." Studies in rats show an increase in number and an altered distribution of the various forms of tubulin, high molecular weight MAPs, and tau which correlate with rat brain development and learning. Both MAP 2 and tau are heat stable, stimulate MT polymerization (lower Cc) and share binding domains on intact MT; they are heterogeneous protein families whose functions are unknown. MAP 2 is concentrated in cell bodies and dendrites of neurons, found in a ratio of MAP 2 to tubulin of one to twelve, occurs in only small amounts in axons and is absent in glia. Conversely, brain tau is largely confined to axons. Both MAP 2 and tau are phosphorylated by cyclic AMP dependent protein kinases, proteins which amplify effects of calcium ions. Intracellular levels of calcium ions, in turn, are regulated by diverse extracellular signals including neurotransmitters, hormones and electrical factors to regulate cell shape, motility, secretory processes and other functions. Thus these MAPs can mediate diverse inputs into the cytoskeleton. Each MAP attached to an MT may function like a "synapse" in a parallel, laterally interconnected network.

Figure 5.14: MAP attachment patterns.
Figure 5.15: MAP attachment patterns.
Figure 5.16: MAP attachment patterns.

Tau heterogeneity varies during brain development and these changes result in the predominance of different tau polypeptides in mature cerebral cortex and cerebellum. Binder, Frankfurter, and Rebhun (1986) propose that MAP 2 binding sites result from genetically determined forms of tubulin which are predominant in neural cell bodies and dendrites, while the tau binding domains are specified by axonal tubulins. They suspect that different tau species appearing during development may herald the cytoskeletal differentiation of unique axonal subpopulations whose MT are committed to slightly different tasks. Variability and responsibilities of MAPs may be extensive. They are functional appendages, structural and communicative links ("arbiters"), and essential integrators of cytoskeletal and cellular function.

Figure 5.17: MAP attachment patterns as part of Koruga (1986) microtubule information code (Chapter 8). With permission from Djuro Koruga.
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