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The cytoskeleton is regulated by a plethora of enzymes that influence thestability and dynamics of cytoskeletal filaments. Molecular motors of thekinesin-8 protein family depolymerise microtubules in a length-dependentmanner, and experimental and theoretical evidence suggest a role for kinesin-8in the dynamic regulation of microtubules. However, so far the detailedmolecular mechanisms how these molecular motors interact with the growingmicrotubule tip remain elusive. Here we investigate two interaction scenariosfor kinesin-8 and the microtubule tip. We give a comprehensive analysis ofregimes where length-regulation is possible and characterise how the stationarylength depends on the biochemical rates and the bulk concentrations of thevarious proteins. For a neutral scenario, where microtubules grow irrespectiveof whether the microtubule tip is occupied by a molecular motor, lengthregulation is possible only for a narrow range of biochemical rates and limitedto small polymerisation rates. In contrast, for an inhibition scenario, wherethe presence of a motor at the microtubule tip inhibits microtubule growth, theregime of length regulation is extremely broad and includes high growth rates.These results also apply to situations where polymerising enzymes, likeXMAP215, and kinesin-8 mutually exclude each other from the microtubule tip. Wealso investigate the stochastic dynamics of the two scenarios. While for theneutral scenario length is tightly controlled, length dynamics is intermittentfor the inhibition scenario and exhibits extended periods of microtubule growthand shrinkage, reminiscent of microtubule dynamic instability. On a broaderperspective, the set of models established in this work quite generallysuggests that mutual exclusion of molecules at the ends of cytoskeletalfilaments is an important factor for filament dynamics and regulation.

Molecular Mechanisms for Microtubule Length Regulation by Kinesin-8 and XMAP215 Proteins