Mutations affecting beta-tubulin folding and degradation

Area of Science:

• Cell Biology
• Molecular Biology
• Protein Folding and Degradation

Background:

• Beta-tubulin is a critical component of microtubules, essential for cell structure and division.
• Mutations in beta-tubulin can lead to altered protein stability and function.
• Cellular mechanisms, including chaperones and proteasomes, regulate protein folding and degradation.

Purpose of the Study:

• To identify cis-acting mutations that destabilize a colcemid-resistant mutant beta-tubulin (D45Y).
• To investigate the folding, assembly, and degradation pathways of these destabilized mutant beta-tubulins.
• To understand the cellular consequences of reduced beta-tubulin levels due to mutant protein instability.

Main Methods:

• Analysis of revertants from a colcemid-resistant Chinese hamster ovary cell line.
• Identification of cis-acting mutations affecting mutant beta-tubulin stability.
• In vivo and in vitro assays to assess tubulin folding, heterodimerization, and proteasomal degradation (using MG132).
• Measurement of cellular tubulin content and assessment of cell growth and survival.

Main Results:

• Four cis-acting mutations (L187R, Y398C, deletion, truncation) were identified that destabilize mutant beta-tubulin.
• Destabilized beta-tubulins fail to form heterodimers, associate with chaperonin CCT, and do not interact with folding cofactors.
• Improperly folded beta-tubulin is degraded by the proteasome, leading to a 30-40% decrease in total cellular tubulin.
• Reduced tubulin content does not impact cell growth or survival.

Conclusions:

• Specific mutations can render mutant beta-tubulin unstable, leading to its failure in productive folding and microtubule incorporation.
• The proteasome efficiently degrades misfolded beta-tubulin, preventing its accumulation.
• Cells can tolerate significant reductions in beta-tubulin levels without compromising viability or growth, suggesting robust cellular regulation.