Background: In fission yeast, an RNA species named meiRNA is specifically required for the promotion of the first meiotic division. To dissect the function of this RNA and its partner RNA-binding protein Mei2, we screened for high-copy-number suppressors of the arrest prior to the first meiotic division caused by loss of meiRNA.
Results: Analysis of one of the suppressors, named ssm4, suggested that it encodes a coiled-coil protein carrying a microtubule-binding motif at its N-terminus. Expression of ssm4 was restricted to cells undergoing meiosis. Disruption of ssm4 affected neither vegetative growth nor conjugation, but resulted in frequent generation of asci carrying less than four spores. Tagged Ssm4 could colocalize with microtubules in mitotic cells, and was seen to localize at spindles during both the first and the second meiotic division. The microtubule-binding motif was essential for the association of Ssm4 with microtubules and for its function during meiosis, but not for the suppression of loss of meiRNA. Ssm4 appeared to possess a potential to migrate to the nucleus.
Conclusions: Ssm4 is a microtubule-colocalizing protein that plays a role specifically in meiosis. Ssm4 appears to modify the structure or the function of nuclear microtubules in order to promote the meiotic nuclear division.