γチューブリンとその関連タンパクの種多様性

Summary: The third tubulin, γ-tubulin, is the most general component of microtubule organizing center(MTOC). Many data have been indicating that γ-tubulin has an essential role for the establishment of microtubule(MT)network. Recent studies have also revealed that γ-tubulin in vivo is not monomeric but exists as a constituent of huge protein complex(es). The most famous complex is γ-tubulin-ring complex(γTuRC)in animal cells. We analysed cytoplasmic γ-tubulin of a fungus, Aspergillus nidulans, and revealed that there are divergent γ-tubulin complexes in this organism. Current knowledge about γ-tubulin biogenesis supports the idea that more than one form of γ-tubulin complexes exist in the cells. Divergency is also seen in γ-tubulin itself. The typical example is γ-tubulin of the budding yeast Saccharomyces cerevisiae. In order to clarify the nature of the divergency, we analysed the sequences of other budding yeast γ-tubulins and found that the divegency spreads among the yeasts. Detailed comparison of the sequences, however, indicated a conservative nature of γ-tubulin; the divergent γ-tubulins still preserve the same or similar amino acids important for rtubulin. The analysis also revealed a feature characteristic to the budding yeast γ-tubulins; a number of amino acids conserved uniquely in them seemed to locate in the molecular surface corresponding to the minus-end of tubulin structure. Furthermore, some of the yeast-specific amino acids seemed to be involved in the nuclear localization of the γ-tubulin. In this review, I also discuss divergencies of γ-tubulin and its related proteins as research objects to know the journey of the MT organizer to its functional maturation.