using highly purified calf brain tubulin bearing [ 8-14c ] guanosine 5'-diphosphate ( gdp ) in the exchangeable nucleotide site and heat-treated microtubule-associated proteins ( both components containing negligible amounts of nucleoside diphosphate kinase and nonspecific phosphatase activities ) , we have found that a significant proportion of exchangeable-site gdp in microtubules can be incorporated directly during guanosine 5'-triphosphate ( gtp ) dependent polymerization of tubulin , without an initial exchange of gdp for gtp and subsequent gtp hydrolysis during assembly .
C0026046
microtubules
C0041348
tubulin