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© The Rockefeller University Press, 0021-9525/1997//717 $5.00
The Journal of Cell Biology, Volume 139, Number 3, , 1997 717-728

Article

Mal3, the Fission Yeast Homologue of the Human APC-interacting Protein EB-1 Is Required for Microtubule Integrity and the Maintenance of Cell Form



Jens D. Beinhauer*, Iain M. Hagan{ddagger}, Johannes H. Hegemann*, and Ursula Fleig*

* Institut für Mikrobiologie und Molekularbiologie, Justus-Liebig-Universität Giessen, 35392 Giessen, Germany and {ddagger} School of Biological Sciences, The University of Manchester, M13 9PT, United Kingdom

Through a screen designed to isolate novel fission yeast genes required for chromosome segregation, we have identified mal3+. The mal3-1 mutation decreased the transmission fidelity of a nonessential minichromosome and altered sensitivity to microtubule-destabilizing drugs. Sequence analysis revealed that the 35-kD Mal3 is a member of an evolutionary conserved protein family. Its human counterpart EB-1 was identified in an interaction screen with the tumour suppressor protein APC. EB-1 was able to substitute for the complete loss of the mal3+ gene product suggesting that the two proteins might have similar functions. Cells containing a mal3 null allele were viable but showed a variety of phenotypes, including impaired control of cell shape. A fusion protein of Mal3 with the Aequorea victoria green fluorescent protein led to in vivo visualization of both cytoplasmic and mitotic microtubule structures indicating association of Mal3 with microtubules. The absence of Mal3 protein led to abnormally short, often faint cytoplasmic microtubules as seen by indirect antitubulin immunofluorescence. While loss of the mal3+ gene product had no gross effect on mitotic spindle morphology, overexpression of mal3+ compromised spindle formation and function and led to severe growth inhibition and abnormal cell morphology. We propose that Mal3 plays a role in regulating the integrity of microtubules possibly by influencing their stability.

Abbreviations used in this paper: APC, adenomatous polyposis coli; FAP, familial adenomatous polyposis; GFP, green fluorescent protein; ORF, open reading frame; MAP, microtubule-associating protein; TBZ, thiabendazole.

Please address all correspondence to Dr. Ursula Fleig, Institut für Mikrobiologie und Molekularbiologie, Frankfurterstrasse 107, 35392 Giessen, Germany. Tel.: +49 641 99 35544. Fax: +49 641 99 35549. E-mail: ursula. fleig{at}bio.uni-giessen.de


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