Tiny Yeast Comet 1-Dependent Polymers Suppress Taxol-Stabilized Microtubule Depolymerization Induced by Ice-Cold CaCl2

Budding yeast Tiny Yeast Comet 1 (Tyc1) was previously identified based on homology to human p31comet. The conserved homologous region is within the previously mapped p31comet Taxol-stabilized microtubule binding domain at the C-terminus of p31comet. The p31comet protein mimics the 3-dimensional shape of the HORMA-domain protein Mitotic Arrest-Deficient 2 (Mad2). Several HORMA-domain proteins have been reported to form polymers. By employing negative staining electron microscopy, we observed that the Tyc1 protein forms comet-tail-shaped polymers in association with Taxol-stabilized microtubules. When associated with microtubules, Tyc1p polymers frequently displayed a braid-like appearance around and off the ends of the microtubules. Analysis of two mutant forms of Tyc1p revealed that the amino acid motifs that are conserved between Tyc1p and human p31comet were essential for robust polymer formation. Tyc1p polymers that were formed in the presence of Mad2p in association with a Mad2-binding motif peptide were able to suppress Taxol-stabilized microtubule depolymerization that was induced by exposure to ice-cold CaCl2. In conclusion, yeast Tyc1p forms polymers that can suppress Taxol-stabilized microtubule depolymerization, potentially yielding an insight into a microtubule-associated function for human p31comet.