Figure 8. A polarized unfurling model for TOG arrays as MT polymerases.

An animation for this model is shown in Video 1. (A) Assembly of yeast MT polymerase dimeric TOG1-TOG2 subunits with four αβ-tubulins into an αβ-tubulin:TOG square. TOG squares diffuse along MT lattices modulated by tubulin C-termini interacting with SK-rich regions. (B) I. TOG square assemblies orient αβ-tubulins in wheel-shaped assemblies at MT plus-ends. II. These assemblies are destabilized upon TOG1-α-tubulin polymerizing onto the exposed β-tubulin at MT plus-ends, releasing TOG1-TOG2 subunits in corner conformations. III. The release of TOG2:αβ-tubulin allows free rotation around TOG1, driving two αβ-tubulins to polymerize. IV. TOG2 dissociates from the newly polymerized αβ-tubulin stabilizing protofilament at the plus-end while TOG1 anchors this αβ-tubulin onto the MT plus-end. V. Straightening of this new protofilament leads to the dissociation of TOG1. The rebinding of TOG1-TOG2 subunits to αβ-tubulins reforms the TOG square assembly and restarts the MT polymerase cycle. Atomic views for states I, II, and III are shown in Figure 7.