Practice question

Polarity of microtubules dictates directionality of transport and regulation by plus end tracking proteins +TIP network. Lattice assembled from head to tail alpha beta dimers creates structurally distinct ends with different chemistry. Minus end shows alpha tubulin outward typically embedded in gamma TuRC within MTOC exchange slow often capped by patronins CAMSAP family. Plus end exposes beta tubulin with GTP binding pocket capable of rapid subunit addition with on rate about 7 per micromolar per second versus minus about half. In vitro growth rates show plus about two to three fold faster elongation also more dynamic undergoing frequent catastrophe and rescue events regulated by EB1 XMAP215. In cells EB family and XMAP215 selectively accumulate at plus ends accelerating growth while minus dynamics dampened and stabilized. This asymmetry enables plus ends to explore cell periphery searching kinetochores and cortical capture sites while minus ends provide stable anchor for force generation during spindle formation and maintain organized radial arrays supporting polarized transport highways.