Solomon F
Biochemistry. 1977 Feb 8;16(3):358-63. doi: 10.1021/bi00622a003.
Calcium ions can inhibit the in vitro assembly of microtubules and, therefore, may play a role in the regulation of microtubule formation in vivo. In order to test the validity of this hypothesis; the interaction between calcium and pruified brain microtubular protein has been investigated by standard binding assays. We have detected and characterized two classes of binding sites for calcium on tubulin, the major component of cytoplasmic microtubules. There is a single high-affinity site per tubulin molecule, characterized by a dissociation constant of 3.2 X 10(-6) M. That site is inhibited by magnesium (k1 = 5 X 10(-5) M) and potassium chloride. There are approximately 16 low-affinity sites which have a dissociation constant of 2.8 X 10(-4) M, and which are also inhibited by potassium chloride. Binding at the low-affinity sites is slightly enhanced by low magnesium concentrations. Both classes of sites are distinguishable from the colchicine binding site, and are apparently also distinct from the vinblastine and guanine nucleotide sites. The characteristics of the calcium binding activity of tubulin are similar to those found for the calcium-binding proteins of sarcoplasmic reticulum. The results are consistent with a physiological role for calcium in the regulation of microtubule assembly.
钙离子能够抑制微管的体外组装，因此，可能在体内微管形成的调节中发挥作用。为了验证这一假说的正确性，通过标准结合试验研究了钙与纯化的脑微管蛋白之间的相互作用。我们已经检测并鉴定了微管蛋白（细胞质微管的主要成分）上两类钙结合位点。每个微管蛋白分子有一个单一的高亲和力位点，其解离常数为3.2×10⁻⁶M。该位点受到镁（k1 = 5×10⁻⁵M）和氯化钾的抑制。大约有16个低亲和力位点，其解离常数为2.8×10⁻⁴M，也受到氯化钾的抑制。低镁浓度会使低亲和力位点的结合略有增强。这两类位点都与秋水仙碱结合位点不同，显然也与长春碱和鸟嘌呤核苷酸位点不同。微管蛋白钙结合活性的特征与肌质网钙结合蛋白的特征相似。这些结果与钙在微管组装调节中的生理作用是一致的。
