Tubulin treadmilling

Though the above mechanism had value in describing the steady-state tubulin flux, Karr and Purich (1979) pointed out that a number of association-dissociation steps can occur for each net protomer addition. They found that the rate of isothermal dilution-induced disassembly of microtubules proceeded at rates 500- to 1000-fold greater than the observed treadmilling rate which itself corresponded to a turnover of about 7% of the tubulin pool/hour (Margolis and Wilson, 1978). Karr and Purich (1979) also proposed that the unidirectional treadmilling model be amended to account for the rapidity of on-off events at the polymer ends, and they offered the alternative mechanism presented here. 

Bergen and Borisy (1980) used the axoneme-hased approach to explore the commitment to treadmilling, and they also found that the efficiency was quite low. An 5 value of 0.07 0.04 was obtained, corresponding to the net addition of 1.6 0.8 tubulin protomers/second. Interestingly, their estimates of the dissociation constants for the two ends were 2.2 0.6 and 3.2 0.6 iiM for the assembly and disassembly ends, respectively. We calculate that this corresponds to only about 0.2 kcal/mol. In a more recent investigation from the same laboratory (Cote and Borisy, 1981), porcine tubulin was found to exhibit an s value of about 0.26, and the rate of the tubulin flux was about 28 protomers/ second. The authors of the latter study suggested that the discrepancy might be accounted for in terms of the need to use MAP-depleted tubulin with the axoneme system to prevent self-nucleation, or in terms of the temperature differences in the two studies. 

Although the lengths of kinetochore and polar microtubules eventually become stable, there continues to be a flow, or treadmilling, of subunits through the microtubules toward the poles. At metaphase, the loss of tubulin subunits at the (—) ends of spindle microtubules Is balanced by the addition of subunits at the (+) ends (Figure 20-37b). The flow of tubulin subunits from kinetochores to the poles can be visualized after a very small pulse of fluorescently labeled tubulin subunits has been microinjected Into a cell (Figure 20-38). Microtubules appear speckled because very few of the subunits are fluores-... 

Kirschner, M.W. (1980). Implications of treadmilling for the stability and polarity of actin and tubulin polymers in vivo. J. Cell Biol. 86, 330-334. 

The consequences of nucleotide hydrolysis for the assembly and dynamits of actin and tubulin polymers are different. The greater on-rate and affinity of the barbed end of F-actin compared to its pointed end are within a range that the net consequence of subunit dynamics is treadmilling, a process by which at steady state there is net addition of actin at the barbed end at the same time that there is net disassembly at the same rate from the pointed end. Thus the average length of a filament remains constant while subunits flux through it from the barbed to the pointed end. 

Assembly of microtubules is asymmetric. At one end, the (+)-end, tubulin dimers associate with the tubule faster than they dissociate, while at the other end, the (-)-end, dissociation occurs more rapidly than association. At equilibrium, the rate of addition of subunits at one end of the microtubule is equal to the rate of loss at the other end the process is referred to as treadmilling , because a given tubulin dimer will pass slowly from the plus to the minus end... 

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