although both ends of a microtubule can gain or lose subunits, one end (called the plus end) polymerizes and depolymerizes at a higher rate than the other end (the minus end). for spindle microtubules, the plus ends are in the center of the spindle, and the minus ends are at the poles. motor proteins that move along microtubules specialize in walking either toward the plus end or toward the minus end; the two types are called plus end-directed and minus end-directed motor proteins, respectively.