( otubule intermediates in the cell. ...)The making and breaking of microtubules

Richard Rodewald, a program officer and cell biologist at the National Institute of General Medical Sciences, which funded the research, noted that "these studies provide a persuasive model, in elegant atomic detail, for how binding of GTP to a specific site in tubulin leads to far-reaching structural changes that drive microtubule growth."

Building a microtubule:

Microtubules are polymers whose basic units are pairs (dimers) of similar but not identical tubulin proteins, dubbed the alpha and beta forms. During polymerization the dimers stack end to end to make a protofilament. About thirteen protofilaments are arranged side by side, extending longitudinally, to form the walls of a cylindrical microtubule.

The so-called minus end of the microtubule grows slowly and is often anchored to a cellular structure. The other end, the plus end, is a hotbed of activity. In the presence of GTP the microtubule's protofilaments acquire more tubulin dimers, and the whole microtubule extends rapidly for many millionths of a meter before suddenly switching off and shrinking again.

Nucleotides like GTP are best known as units of the nucleic acids DNA and RNA, consisting of a base (a letter in the genetic code) plus a sugar and three phosphate groups. But these molecules also regulate enzyme activity and play a crucial role in controlling microtubule growth.

When GTP binds to a particular locus on beta tubulin (the E-site or exchangeable site) polymerization occurs: tubulin dimers pile onto the protofilaments, and the microtubule grows. But through hydrolysis (the word means "water splitting"), GTP readily changes to a different nucleotide, guanosine diphosphate, or GDP. GDP-bound tubulin depolymerizes the microtubule by curling up protofilaments at its plus end and pulling it apart.

Freezing the frenzy:

Although tubulin was known to polymerize in the presence of GTP and depolymerize i
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Source:DOE/Lawrence Berkeley National Laboratory