The size of helical pitch is important for microtubule plus end dynamic instability

Abstract

Microtubule (MT) dynamic instability is a conserved phenomenon underlying essential cellular functions such as cell division, cell migration and intracellular transport, and is a key target of many chemotherapeutic agents. However, it remains unclear how the organization of tubulin dimers at the nanometer scale translates into dynamic instability as an emergent property at the micrometer scale. Tubulin dimers are organized into left-handed helical MT lattice, and most present-day MTs converge at a 1.5 dimer helical pitch that causes a seam in an otherwise symmetric helix. Because presently there are no experimental methods that can precisely manipulate tubulin subunit with sub-dimer resolution, the impact of helical pitch on dynamic instability remains unknown. Here by using stochastic simulations of microtubule assembly dynamics we demonstrate that helical pitch plays essential roles in MT plus end dynamic instability. By systematically altering helical pitch size, one half-dimer at a time, we found that a helical pitch as small as one half-dimer is sufficient to inhibit short-term MT length plateaus associated with diminishing GTP-tubulin cap. Notably, MT plus end dynamics quantitatively scale with the size of helical pitch, rather than being clustered by the presence or absence of helical symmetry. Microtubules with a 1.5 dimer helical pitch exhibit growth and shrinkage phases and undergo catastrophe and rescue similar to experimentally observed microtubules. Reducing helical pitch to 0 promotes rapid disassembly, while increasing it causes microtubules to undergo persistent growth, and it is the 1.5 dimer helical pitch that yields the highest percentage of MTs that undergo alternating growth and shrinkage without being totally disassembled. Finally, although the 1.5 dimer helical pitch is conserved among most present-day MTs, we find that other parameters, such as GTP hydrolysis rate, can partially compensate for changes in helical pitch. Together our results indicate that helical pitch is a determinant of MT plus end dynamic instability and that the evolutionarily conserved 1.5 dimer helical pitch promotes dynamic instability required for microtubule-dependent cellular functions.