Affinity labeling of the exchangeable nucleotide binding site in platelet tubulin

The exchangeable nucleotide binding site of platelet tubulin was labeled with [14C]p-fluorosulfonyl benzoylguanosine (FSBG). FSBG promoted polymerization of tubulin but depolymerization did not occur in the presence of this nucleoside analogue. GTP was able to block FSBG binding to tubulin. [14C]Iodoacetamide-treated tubulin which was first reacted with FSBG was digested with trypsin. The resultant peptides were analyzed by reverse phase high pressure liquid chromatography. One FSBG-labeled peptide could be identified both by its radioactivity and the characteristic UV absorbance spectrum associated with it. This may represent the exchangeable nucleotide site. A second peptide with a distinct nucleotide absorbance peak was found both in FSBG-treated and untreated tubulin preparations. This evidence is suggestive of the non-exchangeable nucleotide binding site.