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J. Cell Biol.,
Volume 139, Number 3, November 3, 1997 717-728
* Institut für Mikrobiologie und Molekularbiologie, Justus-Liebig-Universität Giessen, 35392 Giessen, Germany and 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.
School of
Biological Sciences, The University of Manchester, M13 9PT, United Kingdom
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