Undimerized beta-tubulin is toxic: The microtubule subunit is a heterodimer – unique among cytoskeletal structures. The α- and β-tubulin polypeptides are quite similar to one another. However, small amounts of undimerized β-tubulin are toxic, but cells overexpressing high levels of α-tubulin are viable. Undimerized β-tubulin arises when the balance between β- and α-tubulin expression is disrupted; when the heterodimer dissociates; and when there are defects in the folding of α-tubulin so that it can not form heterodimer. For example, cells deleted for the minor α-tubulin gene TUB3 have ~15 per cent excess β-tubulin, and as a result are supersensitive to microtubule depolymerizing drugs and display increased rates of chromosome mis-segregation. Induced expression of high levels of β-tubulin causes rapid microtubule disassembly and subsequent cell death. Such severe qualitative defects arising from modest quantitative imbalances are a property of other morphogenetic pathways, such as phage assembly. In the case of microtubule assembly, the data suggest that free β-tubulin acts as a dominant interfering protein, competing with the heterodimer for binding to cell components crucial for normal assembly and cell viability.
Rbl2p protects cells from excess β-tubulin: Rbl2p binds β-tubulin to form a heterodimer. Although dispensable for mitotic growth under normal conditions, it is essential in strains with β-tubulin in even modest excess, such as tub3D. The lethality of overexpressed β-tubulin is suppressed by overexpressed Rbl2p. Suppression occurs at Rbl2p levels substantially substoichiometric to the undimerized β-tubulin. Rbl2p may act as a chaperone, transiently binding to β-tubulin to restrict its interactions until it aggregates and is no longer toxic.
Quantitative regulation of tubulin expression: Pac10p is a subunit of the GimC/prefoldin complex that promotes folding of tubulin and other cytoskeletal proteins. Cells deleted for the PAC10 have excess undimerized β-tubulin. Tubulin protein is down regulated in pac10D cells, α-tubulin more than β-tubulin, probably because the role of GimC/prefoldin is more important for α-tubulin. As a result, pac10D cells have microtubule phenotypes and require both RBL2 and TUB3. Surprisingly, pac10D cells downregulate both α- and β-tubulin mRNA levels. We are testing the hypothesis that the relationship between tubulin expression and a defect in protein folding represents feedback regulation of tubulin synthesis. Strains containing combinations of mutations that affect tubulin expression have only ~20% heterodimer levels compared to wild type cells. Nevertheless, these cells grow at rates comparable to wild type cells, but have altered spindle morphology and altered progress through mitosis. These cells provide a new opportunity to identify crucial effectors of microtubule formation and function.
A new step in microtubule morphogenesis. A genetic analysis identified a loss-of-function mutation that protects cells against toxic β-tubulin. PLP1 defines a new, early step in the tubulin folding. A substantial fraction of the β-tubulin made in plp1 cells is not competent to form tubulin heterodimers and also is not toxic. Plp1p acts upstream of the GimC/prefoldin complex and the cytoplasmic chaperonin. Indeed, plp1 suppresses both the microtubule phenotypes and the tubulin downregulation caused by pac10 deletion. We believe that Plp1p facilitates the transfer of nascent β-tubulin polypeptides to the protein folding apparatus, and that it is specific for β-tubulin relative to α-tubulin.
An α-tubulin mutant affects heterodimer stability: The Α-tubulin mutant tub1-724 is one of several mutations that arrest at low temperature with no microtubules. Since microtubule assembly itself is cold-sensitive, such mutations could affect the assembly reaction. However, the tub1-724 mutation destabilizes the heterodimer itself. This mutant α-tubulin binds to β-tubulin with lower affinity than does wild type α-tubulin, and so enhances dissociation of the heterodimer to produce toxic free β-tubulin. The amino acid changed in Tub1-724p is predicted to contact the GTP bound at the interface between α- and β-tubulin in the heterodimer. The properties of this mutant constitute the first structure-function analysis of the heterodimer itself, and provide a tool for studying the mechanisms by which cells manage tubulin polypeptides. This mutant is also suppressed by a mutation in a ribosomal protein. This mutation has numerous microtubule phenotypes, providing another link between microtubule morphogenesis and expression.
Promoting formation of the heterodimer: To identify genes that may affect heterodimer formation or stability, we screened for mutations in genes other than those encoding tubulin but that behave as does tub1-724. This approach identified null alleles in four distinct genes. Each of these non-essential genes had been previously discovered in screens for microtubule phenotypes but their functions had not been characterized. The detailed phenotypes of these mutations indicate that their role likely involves promoting formation of the heterodimer. This hypothesis is supported by the fact that overexpression of a wild type version of three of these genes partially suppresses tub1-724 . The findings suggest that these genes can participate in a catalyzed pathway for heterodimer formation. It can not be the primary pathway for heterodimer formation, since cells deleted for these genes even in pairwise combinations are viable, but they may represent a salvage pathway promoting reassociation of dissociated heterodimers.
An α-tubulin mutant interacts with mitotic checkpoint genes: A second element of the structure-function analysis of tubulin is based on another cold-sensitive α-tubulin allele, tub1-729. This mutant also arrests with no microtubules, like tub1-724. However, the defect in the Tub1-729p is at the attachment between microtubules and the kinetochores. The tub1-729 phenotype is suppressed by overexpression only of a subset of the mitotic checkpoint genes, a fact that distinguishes among their activities. In addition, that suppression does not require downstream elements of the checkpoint pathway. The results demonstrate a structural role for products of some checkpoint genes, and suggest a genetic tool to analyze the details of microtubule-kinetochore attachment.
Prospects: These analyses of cellular regulation of tubulin reveal important quantitative and qualitative regulation that can not be recapitulated by in vitro approaches.
Our next goals are to identify the mechanisms by which cells regulate tubulin expression; to characterize the earliest steps in microtubule morphogenesis, those that lead to heterodimer formation; to pursue a structure-function analysis of the tubulins; and to understand the mechanism of β-tubulin lethality.
Lacefield, S. and F. Solomon. A novel step in β-tubulin folding is important for heterodimer formation in S. cerevisiae. Genetics. In press.
Abruzzi, K. , M. Magendantz, and F. Solomon. An α-tubulin mutant demonstrates distinguishable functions among the spindle assembly checkpoint genes in S. cerevisiae. Genetics. 161: 983-994 (2002).
Abruzzi, K. , A. Smith, W. Chen and F. Solomon. Protection from free β-tubulin by the β-tubulin binding protein Rbl2p. Molecular and Cellular Biology. 22:138-147 (2002).
Fleming, J. A. , L. R. Vega and F. Solomon. Function of tubulin binding proteins in vivo: A salvage pathway for dissociated tubulin heterodimers. Genetics. 156: 69-80 (2000).
Vega, L. R, J. Fleming and F. Solomon. An α-tubulin mutant destabilizes the heterodimer: phenotypic consequences and interactions with tubulin binding proteins. Molecular Biology of the Cell 9: 2349-2360 (1998).