EVIDENCE THAT THE TIGHTLY BOUND MAGNESIUM IN TUBULIN IS ASSOCIATED WITH THE N-SITE GTP

In an attempt to determine whether the tightly bound Mg2+ found in purified tubulin in associated with the N-site GTP or the E-site GDP or GTP, we removed the E-site nucleotide by several means: (i) alkaline phosphatase treatment; (ii) displacement using excess GMPPCP; and (iii) polymerizing tubulin in the presence of alkaline phosphatase and non-hydrolyzable analogues. The Mg2+ content remained equal to about 1 mol/mol tubulin under conditions where denaturation did not occur. Moreover, the Mg/GTP ratio always remained equal to 1. These results indicate that the Mg2+ is associated with the N-site GTP.