Effects of Spermine on Microtubule Structures

Abstract

Spermine is a tetravalent polyamine present at millimolar concentration in all eukaryotic cells. The adsorption of cationic spermine on anionic microtubules (MTs) surface may have a potential regulatory role on the complex dynamics of MTs. Experiments have shown that polyamines such as spermine can facilitate the polymerization of tubulin at physiological concentration, including the formation of nuclei or growth of MTs. The underlying molecular mechanism is thought to be based on the bridging interaction of spermine with the highly negative C-terminal tail on the tubulin surface that helps bind tubulin together. Recent experiments also show that bundles of taxol-stabilized MTs undergo a shape transformation to bundles of inverted tubulin tubules (ITTs) at high concentrations of spermine, where the outside surface of an ITT corresponds to the inner surface of a regular MT. Furthermore, the longitudinal (lateral) bonds in a regular MT along its axis (circumference) are transformed into lateral (longitudinal) bonds in an ITT along its circumference (axis). However, the molecular mechanism responsible for the shape transformation is unclear. We performed all atom molecular dynamics simulation on taxol-stabilized MT sheets containing two protofilaments with and without spermine in an explicit water solvent. Our simulations have identified important regions on the MT surface where spermine binds and the influence of spermine binding on the structure of tubulin proteins and tubulin-tubulin contacts in MTs. In contrast to Taxol, spermine-binding seems to decrease the flexibility of tubulin proteins, resulting in weaker tubulin-tubulin contacts. Our results point to a possible molecular mechanism of how spermine disrupts the contact between protofilaments and facilitates the bending of protofilaments into inverted rings and eventually inverted tubules.