Bergen L G, Borisy G G
J Cell Biol. 1980 Jan;84(1):141-50. doi: 10.1083/jcb.84.1.141.
Microtubules are polar structures, and this polarity is reflected in their biased directional growth. Following a convention established previously (G. G. Borisy, 1978, J. Mol. Biol. 124:565--570), we define the plus (+) and minus (-) ends of a microtubule as those equivalent in structural orientation to the distal and proximal ends, respectively, of the A subfiber of flagellar outer doublets. Rates of elongation were obtained for both ends using flagellar axonemes as seeds and porcine brain microtubule protein as subunits. Since the two ends of a flagellar seed are distinguishable morphologically, elongation of each end may be analyzed separately. By plotting rates of elongation at various concentrations of subunit protein, we have determined the association and dissociation rate constants for the plus and minus ends. Under our conditions at 30 degrees C, the association constants were 7.2 X 10(6) M-1 s-1 and 2.25 X 10(6) M-1 s-1 for the plus and minus ends, respectively, and the dissociation constants were 17 s-1 and 7 s-1. From these values and Wegner's equations (1976, J. Mol. Biol. 108:139--150), we identified the plus end of the microtubule as its head and calculated "s," the head-to-tail polymerization parameter. Surprisingly small values (s = 0.07 +/- 0.02) were found. The validity of models of mitosis based upon head-to-tail polymerization (Margolis et al., 1978, Nature (Lond.) 272:450--452) are discussed in light of a small value for s.
微管是极性结构，这种极性反映在它们有偏向性的定向生长中。按照先前确立的惯例（G.G.博里西，1978年，《分子生物学杂志》124:565 - 570），我们将微管的正（+）端和负（-）端分别定义为在结构取向上与鞭毛外双联体A亚纤维的远端和近端等效的末端。以鞭毛轴丝为种子，猪脑微管蛋白为亚基，分别获得了两端的伸长速率。由于鞭毛种子的两端在形态上是可区分的，所以可以分别分析每一端的伸长情况。通过绘制不同亚基蛋白浓度下的伸长速率曲线，我们确定了正端和负端的缔合和解离速率常数。在我们30摄氏度的条件下，正端和负端的缔合常数分别为7.2×10⁶ M⁻¹ s⁻¹和2.25×10⁶ M⁻¹ s⁻¹，解离常数分别为17 s⁻¹和7 s⁻¹。根据这些值和韦格纳方程（1976年，《分子生物学杂志》108:139 - 150），我们将微管的正端确定为其头部，并计算了“s”，即头对头聚合参数。结果发现该值出奇地小（s = 0.07 ± 0.02）。鉴于s值较小，我们讨论了基于头对头聚合的有丝分裂模型（马戈利斯等人，1978年，《自然》（伦敦）272:450 - 452）的有效性。
