Practice question

During mitosis interphase long stable microtubules must be extensively remodeled into highly dynamic spindle capable of search and capture chromosome alignment and segregation. Two coordinated regulatory shifts occur: dramatic phosphorylation of stabilizing MAPs such as MAP2 MAP4 and tau by CDK1 Aurora B and other mitotic kinases reduces their affinity several fold detaching them from lattice exposing filament to remodeling enzymes and increasing turnover; concurrently microtubule severing AAA ATPases katanin spastin and fidgetin activated via phosphorylation recruitment and release from sequestration. Katanin forms hexameric ring that uses ATP hydrolysis to pull tubulin C terminal tail through pore breaking lateral contacts generating fragments. MAP2 itself not severing enzyme but mitotic phosphorylation of MAP2 permits severing enzymes access to lattice. Result increased creation of short fragments dynamic ends increased catastrophe frequency. Transition ensures spindle size scaling chromosome capture and segregation illustrating regulated switch from stabilization to severing dominance crucial for mitotic progression. Additional regulatory inputs including phosphorylation, small GTPases, and cargo adaptors fine tune filament assembly stability and motor activity matching cellular demands during division, migration, and mechanical stress responses efficiently.