Tubulins contain a glycine-rich loop, that has been implicated in micr
otubule dynamics by means of an intramolecular interaction with the ca
rboxy-terminal region. As a further extension of the analysis of the r
ole of the carboxyterminal region in tubulin folding we have mutated t
he glycine-rich loop of tubulin subunits. An alpha-tubulin point mutan
t with a T-150-->G substitution (the corresponding residue present in
beta-tubulin) was able to incorporate into dimers and microtubules. On
the other hand, four beta-tubulin point mutants, including the (G148)
-->T substitution, did not incorporate into dimers, did not release mo
nomers, but were able to form C-900 and C-300 complexes (intermediates
in the process of tubulin folding). Three other mutants within this r
egion (which approximately encompasses residues 137-152) were incapabl
e of forming dimers and C-300 complexes but gave rise to the formation
of C-900 complexes. These results suggest that tubulin goes through t
wo sequential folding states during the folding process, first in asso
ciation with TCP1-complexes (C-900) prior to the transfer to C-300 com
plexes. It is this second step that for tubulin folding and assembly.