Assembly of microtubules from preformed, ring‐shaped protofilaments and 6‐s tubulin

Abstract Tubulin obtained from disassembly of microtubules at 0°C exists in two forms: 6‐S tubulin and a larger, curved or ring‐shaped filament. These two forms have been separated chromat ographically and their roles in assembly examined. The purified rings reassemble to microtubules with high efficiency by uncoiling and straightening out, to be incorporated directly as protofilaments in the microtubule wall, and are thus identified as preformed protofilaments. Purified 6‐S tubulin has not been observed to reassemble into microtubules by itself but will contribute to assembly when mixed with rings. Addition of glycerol at 0°C induces the 6‐S tubulin to form rings, and the treated fraction will then reassemble to microtubules. Electron microscope observations indicate that assembly begins with the formation and growth of an incomplete microtubule wall. This wall grows wider by the addition of new protofilaments until the intact, circular microtubule, with 13 protofilaments, is formed. It is suggested here that growth of this wall from individual 6‐S tubulin subunits may be energetically unfavorable. The direct incorporation of preformed protofilaments may be much more favorable, in which case the rings would be required for this initial stage of assembly.