MAL3, THE FISSION YEAST HOMOLOG OF THE HUMAN APC-INTERACTING PROTEIN EB-1 IS REQUIRED FOR MICROTUBULE INTEGRITY AND THE MAINTENANCE OF CELLFORM

Through a screen designed to isolate novel fission yeast genes require d for chromosome segregation, we have identified mal3(+). The mal3-1 m utation decreased the transmission fidelity of a nonessential minichro mosome and altered sensitivity to microtubule-destabilizing drugs, Seq uence analysis revealed that the 35-kD Mal3 is a member of an evolutio nary conserved protein family. Its human counterpart EB-1 was identifi ed in an interaction screen with the tumour suppressor protein APC. EB -1 was able to substitute for the complete loss of the mal3(+) gene pr oduct suggesting that the two proteins might have similar functions. C ells containing a mal3 null allele were viable but showed a variety of phenotypes, including impaired control of cell shape. A fusion protei n of Mal3 with the Aequorea victoria green fluorescent protein led to in vivo visualization of both cytoplasmic and mitotic microtubule stru ctures indicating association of Mal3 with microtubules. The absence o f Mal3 protein led to abnormally short, often faint cytoplasmic microt ubules as seen by indirect antitubulin immunofluorescence. While loss of the mal3(+) gene product had no gross effect on mitotic spindle mor phology, overexpression of mal3(+) compromised spindle formation and f unction and led to severe growth inhibition and abnormal cell morpholo gy. We propose that Mal3 plays a role in regulating the integrity of m icrotubules possibly by influencing their stability.