Microtubule plus end

Rogers, S. L., Wiedemann, U., Hacker, U., Turck, C., and Vale, R. D. (2004). Drosophila RhoGEF2 associates with microtubule plus ends in an EBl-dependent manner. 

Orbit/MAST proteins, also known as CLIP-associated proteins (CLASPs), are involved in the regulation of microtubule dynamics and bind to microtubule plus ends via CLIP115 or CLIP170. Active CLASP suppresses microtubule assembly and axon outgrowth (Lee et al., 2004), whereas activated adenomatous polyposis coli protein (APC see below) promotes microtubule assembly and axon outgrowth. 

Fig. 11. Model of EB1/APC complex. Summary of what is known about different regions of the two polypeptides EB1 and APC and their interactions. The globular N-terminal CH-domain of EB1 is shown in Figure 7C. The complete molecule, which binds preferentially to microtubule plus ends, dimerizes via a stretch of coiled-coil. This region also binds to APC, thus targeting APC to the plus ends, too. The shape of APC is purely hypothetical.

Lee, H., Engel, U., Rusch, J., Scherrer, S., Sheard, K., and Van Vactor, D. (2004). The microtubule plus end tracking protein Orbit/MAST/CLASP acts downstream of the tyrosine kinase Abl in mediating axon guidance. Neuron 42, 913-926. 

Schuyler, S. C., and Pellman, D. (2001). Microtubule plus-end-tracking proteins The end is just the beginning. Cell 105, 421-424. 

Vaughan, P. S., Miura, P., Henderson, M., Byrne, B., and Vaughan, K. T. (2002). A role for regulated binding of pl50Glued to microtubule plus ends in organelle transport. J. Cell Biol. 158, 305-319. 

Xiang X. LISl at the microtubule plus end and its role in dynein-mediated nuclear migration. J Cell Biol 2003 160(3) 289-290. 

Fig. 1 Schematic representation of a neuron showing neurite morphology. Arrows indicate orientations of microtubules, plus-end outward in the axon, and microtubules with mixed polarization in dendrites. Cytoskeletal properties specific for axon or dendrite are depicted in the box below...

Because all tubulin dimers in a microtubule are oriented similarly, microtubules are polar structures. The end of the microtubule at which growth occurs is the plus end, and the other is the minus end. Microtubules in vitro carry out a GTP-dependent process called treadmiUing, in which tubulin dimers are added to the plus end at about the same rate at which dimers are removed from the minus end (Figure 17.3). 

The axoneme consists of a cylinder of nine outer doublets of fused microtubules and a pair of discrete central microtubules (commonly referred to as the 9 + 2 arrangement of microtubules). The outer doublets each consist of a complete A-microtubule and an incomplete B-microtubule, the deficiency in the wall of the latter being made up by a sharing of wall material with the former. The tip of the axoneme contains the plus ends of all of the constituent microtubules. Two curved sidearms, composed of the MAP protein dynein, are attached at regular intervals to the A-microtubules of each fused outer doublet (Figures 1 and 2). 

Radial arrays are best exemplified by mitotic half-spindles, which have a central MTOC, the centrosome. The centrosome consists of two centrioles (which are homologous with the basal body) surrounded by dense pericentriolar material (Kalt and Schliwa, 1993). In plant cells, the MTOC of the mitotic spindle consists of dense material only without centrioles. The plus ends of microtubules of the mitotic spindle are directed toward the equator of the cell. Some are free, and others attach to kinetochores on chromatids (see Figure 4). 

Newport-. The way that spindles and microtubules are normally reoriented is by the stabilization of dynamic instability at the plus-end of the microtubule. So if microtubules were to embed in this apically localized complex, they would effectively be capped and this would reorient the spindle. One would expect that this would happen to the centriole prior to mitosis, so that the interphase microtubules would be stabilized at that location as well. Are you saying this doesn t happen If it doesn t happen, perhaps Cdc2 is necessary to activate this apical region for stabilizing plus ends, and this would explain why it rocks about. Are any of these molecules potential candidates for capping microtubules at the plus-end, for instance ... 

Cytoplasmic dyneins may have multiple roles in the neuron. The identification of kinesin as a plus-end directed microtubule motor suggested that it is involved in anterograde transport but left the identity of the retrograde motor an open question. Since flagellar dynein was known to be a minus-end-directed motor, interest in cytoplasmic dyneins was renewed. Identification of the cytoplasmic form of dynein in nervous tissue came as an indirect result of the discovery of kinesin. 

In resting neutrophils it is estimated that there are about 11-23 microtubules per cell, with a diameter of approximately 25 nm and a wall width of 5 nm. They are long, tubular structures made by the helical formation of tubulin molecules, which are either a- or /3-subunits, each with a relative molecular mass of 55 kDa (Fig. 4.9). Each subunit is present in equimolar amounts in a tubulin molecule, and these subunits exist as dimers of one a- and one /3-subunit. Because microtubules are polar, growth of the fibre is biased towards one end, termed the plus end. A number of microtubule-associated proteins (MAPs) affect the dynamic shape of the microtubule, and in the resting neutrophil about 35-40% of the tubulin pool is assembled, whilst the remainder can be assembled very rapidly after cell stimulation. 

With respect to a solvent, the overall solvation capability for solutes. 2. A property of bodies or systems that have a distinct direction i.e., that have different or opposing physical properties or characteristics at different points. For example, an amino acid sequence in a polypeptide has polarity in that there is an amino end and a carboxyl end of the sequence. Similarly, microtubules and actin filaments have plus (+)-ends and minus (-)-ends that establish directionality for cellular and intracellular locomotion. 3. The state in which there is either a positive or negative aspect relative to the two poles of a magnet or to electrification. 4. Attraction toward an object or attraction in a specific direction. 5. In mathematics, the positive or negative sign of numbers. 

Figure 7-33 Stereoscopic ribbon diagram of the tubulin dimer with a-tubulin with bound GTP at the top and P-tubulin with bound GDP at the bottom. The p-tubulin subunit also contains a bound molecule of taxotere (see Box 7-D) which is labeled TAX. This model is based upon electron crystallography of zinc-induced tubulin sheets at 0.37-nm resolution and is thought to approximate closely the packing of the tubulin monomers in microtubules.315 The arrow at the left points toward the plus end of the microtubule. Courtesy of Kenneth H. Downing.

Other major differences between kinesins and myosin II heads involve kinetics180 181 and processivity.173 Dimeric kinesin is a processive molecule. It moves rapidly along microtubules in 8-nm steps but remains attached.182 1823 Myosins V and VI are also proces-sive1 83, a3e but myosin II is not. It binds, pulls on actin, and then releases it. The many myosin heads interacting with each actin filament accomplish muscle contraction with a high velocity in spite of the short time of attachment. Ned and Kar3 are also nonprocessive and slower than the plus end-oriented kinesins.184... 

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