The majority of the Saccharomyces cerevisiae septin complexes do not exchange guanine nucleotides

We show here that affinity-purified Saccharomyces cerevisiae septin complexes contain stoichiometric amounts of guanine nucleotides, specifically GTP and GDP. Using a N-15-dilution assay read-out by liquid chromatography-tandem mass spectrometry, we determined that the majority of the bound guanine nucleotides do not turn over in vivo during one cell cycle period. In vitro, the isolated S. cerevisiae septin complexes have similar GTP binding and hydrolytic properties to the Drosophila septin complexes (Field, C. M., al-Awar, O., Rosenblatt, J., Wong, M. L., Alberts, B., and Mitchison, T. J. (1996) J. Cell Biol. 133, 605-616). In particular, the GTP turnover of septins is very slow when compared with the GTP turnover for Ras-like GTPases. We conclude that bound GTP and GDP play a structural, rather then regulatory, role for the majority of septins in proliferating cells as GTP does for alpha-tubulin.