Microtubule assembly/disassembly.

<p>Each longitudinal protofilament of an MT is made up of α and β tubulin subunit heterodimers. Assembly of MTs depends on addition of ‘GTP-tubulin’. Top, right: blue dimers with both exchangeable and non-exchangeable sites occupied by GTP will continue to add to assembling MTs. Bottom, right: dimers at MT edge which hydrolyze GTP to GDP flex and separate protofilaments from MT, curved protofilaments resembling ‘rams horns’.