UNIQUE TUBULIN ASSEMBLIES IN THE PRESENCE OF SPERMINE
The ability of tubulin to assemble into different morphologies is essential for cellular activities. The way that cellular factors regulate the complex construction of tubulin assemblies is still elusive. Spermine, an endogenous polyamine, is a central regulator of the formation of tubulin assemblies. Analyzing tubulin assemblies in the presence of spermine and understanding the mechanism of their formation, may contribute to comprehend their multifunctional nature. Here we show that spermine induced tubulin to assemble in vitro into hierarchical architectures, based on a unique conical frustum tubulin (CFT) subunit. Using solution X-ray scattering and cryogenic transmission electron microscopy, we showed that with progressive increase of spermine concentration, tubulin-dimers assembled into tubulin-rings, CFTs, CFT-tubes, bundles of antiparallel CFT-tubes, and eventually to hexagonal bundles of inverted tubulin tubules (ITTs). Time-resolved experiments revealed that tubulin assemblies formed at low spermine concentrations, were precursors of assemblies formed at higher concentrations. The results provide insight into the regulatory role that spermine has on the architecture of cytoskeletal networks and form the basis for understanding the fundamental interactions that control the structures of protein-based arrays.