Formation and Function of the Rbl2p–β-Tubulin Complex

The yeast protein Rbl2p suppresses the deleterious effects of excess β-tubulin as efficiently as does α-tubulin. Both in vivo and in vitro, Rbl2p forms a complex with β-tubulin that does not contain α-tubulin, thus defining a second pool of β-tubulin in the cell. Formation of the complex depends upon the conformation of β-tubulin. Newly synthesized β-tubulin can bind to Rbl2p before it binds to α-tubulin. Rbl2p can also bind β-tubulin from the α/β-tubulin heterodimer, apparently by competing with α-tubulin. The Rbl2p–β-tubulin complex has a half-life of ∼2.5 h and is less stable than the α/β-tubulin heterodimer. The results of our experiments explain both how excess Rbl2p can rescue cells overexpressing β-tubulin and how it can be deleterious in a wild-type background. They also suggest that the Rbl2p–β-tubulin complex is part of a cellular mechanism for regulating the levels and dimerization of tubulin chains.

酵母蛋白Rbl2p可与α微管蛋白（α-tubulin）一样高效地拮抗过量β微管蛋白（β-tubulin）所引发的有害效应。无论是在体内（in vivo）还是体外（in vitro）条件下，Rbl2p均可与β微管蛋白形成不含α微管蛋白的复合物，这一现象定义了细胞内第二池β微管蛋白。该复合物的形成依赖于β微管蛋白的构象状态。新合成的β微管蛋白在与α微管蛋白结合之前，即可与Rbl2p结合。Rbl2p还可结合α/β微管蛋白异二聚体（α/β-tubulin heterodimer）中的β微管蛋白，其机制似乎是通过与α微管蛋白竞争结合位点。Rbl2p–β微管蛋白复合物的半衰期约为2.5小时，其稳定性低于α/β微管蛋白异二聚体。本实验结果既阐明了过量表达的Rbl2p为何能够挽救β微管蛋白过表达的细胞，也解释了其在野生型细胞背景中为何会产生有害效应。研究结果还提示，Rbl2p–β微管蛋白复合物是调控微管蛋白亚基水平与二聚化过程的细胞调控机制的组成部分。