Septins are conserved GTP-binding cytoskeletal proteins that polymerize into filaments by end-to-end joining of heterooligomeric complexes. In human cells, both hexamers and octamers exist, and crystallography studies predicted the order of the hexamers to be SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7, while octamers are thought to have the same core, but with SEPT9 at the ends. However, based on this septin organization, octamers and hexamers would not be expected to co-polymerize due to incompatible ends. Here we isolated hexamers and octamers of specific composition from human cells and show that hexamers and octamers polymerize individually and, surprisingly, with each other. Binding of the BD3 domain of Borg3 results in distinctive clustering of each filament type. Moreover, we show that the organization of hexameric and octameric complexes is inverted compared to its original prediction. This revised septin organization is congruent with the organization and behavior of yeast septins suggesting that their properties are more conserved than was previously thought.

Septins 是保守的 GTP 结合细胞骨架蛋白，通过异寡聚复合物的端到端连接聚合成细丝。在人类细胞中，六聚体和八聚体都存在，晶体学研究预测六聚体的顺序是 SEPT7-SEPT6-SEPT2-SEPT2-SEPT6-SEPT7，而八聚体被认为具有相同的核心，但末端是 SEPT9。然而，基于这种 septin 组织，由于末端不相容，八聚体和六聚体预计不会共聚。在这里，我们从人体细胞中分离出特定组成的六聚体和八聚体，并表明六聚体和八聚体单独聚合，而且令人惊讶的是，它们彼此聚合。Borg3 的 BD3 域的结合导致每种细丝类型的独特聚类。此外，我们表明，与最初的预测相比，六聚体和八聚体复合物的组织是颠倒的。这种修改后的 septin 组织与酵母 septin 的组织和行为一致，表明它们的特性比以前认为的更保守。