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* Department of Biochemistry, New York University Medical Center, New York 10016; The production of native Department of Biological Sciences,
Stanford University, Stanford, California 94305; and § Flanders Interuniversity Institute for Biotechnology and University of
Ghent, B-9000 Ghent, Belgium
/
tubulin heterodimer in vitro depends on the action of cytosolic
chaperonin and several protein cofactors. We previously showed that four such cofactors (termed A, C, D,
and E) together with native tubulin act on -tubulin folding intermediates generated by the chaperonin to
produce polymerizable tubulin heterodimers. However, this set of cofactors generates native heterodimers
only very inefficiently from -tubulin folding intermediates produced by the same chaperonin. Here we describe the isolation, characterization, and genetic analysis of a novel tubulin folding cofactor (cofactor B) that
greatly enhances the efficiency of -tubulin folding in
vitro. This enabled an integrated study of - and
-tubulin folding: we find that the pathways leading to
the formation of native - and -tubulin converge in
that the folding of the subunit requires the participation of cofactor complexes containing the subunit and
vice versa. We also show that sequestration of native
-or -tubulins by complex formation with cofactors
results in the destabilization and decay of the remaining
free subunit. These data demonstrate that tubulin folding cofactors function by placing and/or maintaining
-and -tubulin polypeptides in an activated conformational state required for the formation of native /
heterodimers, and imply that each subunit provides information necessary for the proper folding of the other.
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