The group identified Fin1 in a screen for interaction partners of the 14-3-3 family of regulatory proteins. In nondividing cells, Fin1 is undetectable, whereas in small budded cells it localizes to the nucleus. As the cell cycle progresses, Fin1 forms a filament between the nuclei of dividing cells, extending between the spindle pole bodies. Fin1 also forms an intermediate filament in vitro and seems to act independently of tubulin. “It is the first time that someone has found a filament between spindle poles that is different from microtubules,” says van Heusden.
The only other known yeast intermediate filament is the Mdm1 filament involved in mitochondrion inheritance. The function of Fin1 is unknown, as the fin1 mutant is viable. van Heusden speculates that the filament may organize proteins in the nucleus during cell division, and he plans to identify synthetic lethal mutants to address this possibility.
Unlike mutation of Fin1, protein overexpression is lethal in haploid cells. The filamentous structures that develop in these cells and their detrimental effects on cell growth resemble the accumulation of the tau protein in Alzheimer's disease. Fin1 may therefore provide a model for the study of tau filament formation. ▪