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© The Rockefeller University Press, 0021-9525/1997//821 $5.00
The Journal of Cell Biology, Volume 138, Number 4, , 1997 821-832

Article

Tubulin Subunits Exist in an Activated Conformational State Generated and Maintained by Protein Cofactors



Guoling Tian*, Sally A. Lewis*, Becket Feierbach{ddagger}, Timothy Stearns{ddagger}, Heidi Rommelaere§, Christophe Ampe§, and Nicholas J. Cowan*

* Department of Biochemistry, New York University Medical Center, New York 10016; {ddagger} Department of Biological Sciences, Stanford University, Stanford, California 94305; and § Flanders Interuniversity Institute for Biotechnology and University of Ghent, B-9000 Ghent, Belgium

The production of native {alpha}/β 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 β-tubulin folding intermediates generated by the chaperonin to produce polymerizable tubulin heterodimers. However, this set of cofactors generates native heterodimers only very inefficiently from {alpha}-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 {alpha}-tubulin folding in vitro. This enabled an integrated study of {alpha}- and β-tubulin folding: we find that the pathways leading to the formation of native {alpha}- and β-tubulin converge in that the folding of the {alpha} subunit requires the participation of cofactor complexes containing the β subunit and vice versa. We also show that sequestration of native {alpha}-or β-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 {alpha}-and β-tubulin polypeptides in an activated conformational state required for the formation of native {alpha}/β heterodimers, and imply that each subunit provides information necessary for the proper folding of the other.

Abbreviations used in this paper: c-cpn, cytosolic chaperonin; mt-cpn, mitochondrial chaperonin.

Please address all correspondence to Nicholas J. Cowan, Department of Biochemistry, New York University Medical Center, 550 First Avenue, New York, NY 10016. Tel.: (212) 263-5809. Fax: (212) 263-8166.


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