Protist tubulins: new arrivals, evolutionary relationships and insights to cytoskeletal function.

The protists exhibit probably the most extravagant expression of microtubule-containing structures found in any organism. These structures--flagella, cilia, axostyles, spindles and a veritable constellation of microtubule bundles and cortical arrays--provide shape, form, motility, anchorage and apparatuses for feeding. The cytoskeletal structures have a precise order (i.e. size, position and number) that must be replicated and segregated with fidelity at each division, some components being inherited conservatively and others semi-conservatively. Intriguingly, it is now apparent that much of the high-order organisation, which was recognised and described by light and electron microscopy during the last century, is a reflection of molecular polarities set by assembly of constituent proteins. Tubulins and microtubules lie at the heart of this morphogenetic pattern.