Ornithine .delta.-aminotransferase (OAT) facilitates microtubule assembly in addition to its aminotransferase activity in mitochondria. N-terminal proteolysis of the first 17 amino acids of OAT block its transport to the mitochondria. The resultant truncated protein (OATC) forms specific complexes with mitotic spindle promoting proteins such as Eg5 and takes on a Ran-dependent spindle-assembly activity. Methods and compositions for inhibiting mitotic spindle assembly in a cell by specifically inhibiting OAT, and methods for screening for inhibitors of (1) the spindle-assembly function of OAT, (2) the protease that N-truncates OAT, (3) the OAT/RanGTP association and (4) the OAT/Eg5 association are disclosed.