What is the function of augmin in microtubule assembly?
Spindle microtubule amplification cannot rely solely on centrosomes especially in large oocytes acentriolar spindles and plant cells lacking centrioles. Augmin complex discovered as eight subunit module HAUS in humans provides branching nucleation pathway essential for spindle density. Mechanism: augmin binds along side wall of preexisting mother microtubule through its microtubule binding subunits HAUS6 and HAUS8 then recruits gamma tubulin ring complex via NEDD1 and GCP WD domain adaptor through direct interaction. Daughter microtubule nucleates at shallow angle about 35 degrees retaining same polarity as mother growing outward creating branched array increasing density without new MTOC requirement. This augments kinetochore fibers and bridging fibers maintaining robust spindle. Depletion leads to weak spindles reduced microtubule number elongated spindles chromosome mis segregation and mitotic delay. Thus augmin binds existing microtubules and recruits gamma TuRC cooperatively distinct from direct GTP binding or depolymerization promoting activities previously described for MAPs. 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.
Ref: Petry et al., Cell 2013 – Augmin binds existing microtubules and recruits gamma TuRC for branching nucleation.