Characterization of AIR9 - a novel plant microtubule-associated protein that marks the division plane

植物细胞不会像动物细胞那样四处爬行。取而代之的是，细胞是根据细胞之间铺设交叉墙的平面在组织和器官中组织起来的。奇怪的是，在质膜内表面形成的一圈微管(称为前相带)实际上发生分裂之前，就预测到了分裂平面。细胞核分裂后，在两个子核之间放置一个新的交叉壁，并在由前相带预测的确切位置连接到亲本细胞壁。然而，这条带在分裂前就消失了，我们不知道在跨壁形成之前，预测的细胞分裂平面是如何“记忆”的。这条带可能会在细胞表面留下一个标记(S)--一个“分子潮汐标记”？在寻找可能结合并调节植物微管的新蛋白质的过程中，我们发现了一种名为AIR9的大蛋白质。通过将其连接到荧光标记，我们可以看到这种蛋白质装饰前期带，然后随着细胞开始分裂，随着带的微管一起消失。新的交叉壁从细胞中心开始，向外生长，直到它接触亲本细胞壁。当这种跨壁接触到前相带的前一位置时，我们可以看到AIR9荧光信号重新出现在细胞表面。这表明AIR9结合蛋白被微管的前期带留下，当跨壁完成其预测的旅程时，该蛋白被第二次识别。因此，AIR9在植物细胞分裂的这一关键阶段提供了首批处理之一。我们将看看AIR9是否直接与微管结合，或者需要另一个伙伴。我们已经发现AIR9很可能与两种有趣的蛋白质结合(一种参与细胞壁的合成，另一种是沿着微管运动的马达)，我们建议确认这一点。其他实验的目的将是检查这种蛋白质的突变，以及通过实验诱导细胞产生过多或过少的蛋白质。我们将改变蛋白质，使其调节部位不能被细胞修改(通过磷酸化)，看看是否可以使蛋白质在细胞分裂周期的不适当阶段发挥作用。这些处理产生的缺陷应该为AIR9如何在植物中发挥作用提供有价值的见解。

Plant cells do not crawl around like animal cells. Instead, cells are organized in tissues and organs according to the plane in which cross-walls are laid down between cells. Curiously, the division plane is predicted before division actually takes place by a ring of microtubules (called the preprophase band) that forms at the inner surface of the plasma membrane. After the nucleus has divided, a new cross-wall is put down between the two daughter nuclei and this attaches to the parental cell wall at the exact site forecast by the preprophase band. However, the band disappears before division and we have no idea of how the predicted plane of cell division can be 'memorized' in the period until the cross-wall forms. Possibly the band leaves behind a marker(s) on the cell surface - a 'molecular tidemark'? In a search for novel proteins that might bind, and hence regulate, plant microtubules, we discovered a large protein named AIR9. By coupling it to a fluorescent marker, we can see that this protein decorates the preprophase band and then disappears along with the band's microtubules as the cell starts to divide. The new cross-wall starts off in the centre of the cell and grows outwards until it touches the parental cell wall. The moment this cross-wall contacts the former site of the preprophase band we can see that the fluorescent AIR9 signal reappears upon the cell surface. This suggests that an AIR9-binding protein was left behind by the preprophase band of microtubules and is recognized for a second time when the cross-wall completes its predicted journey. AIR9 therefore offers one of the first handles on this crucial phase in plant cell division. We will see if AIR9 binds directly to microtubules or needs another partner. We have already found that AIR9 is likely to bind to two interesting proteins (one involved in cell wall synthesis, the other a motor that travels along microtubules) and we propose to confirm this. Other experiments will be aimed at examining mutations in this protein as well as experimentally inducing cells to produce too much or too little of the protein. We will alter the protein so that its regulatory sites cannot be modified by the cell (by phosphorylation) to see if the protein can be made to function at inappropriate stages of the cell's division cycle. The defects that arise from these treatments should provide valuable insights into how AIR9 functions in plants.