THE YEAST DYNACTIN COMPLEX IS INVOLVED IN PARTITIONING THE MITOTIC SPINDLE BETWEEN MOTHER AND DAUGHTER CELLS DURING ANAPHASE-B

Although vertebrate cytoplasmic dynein can move to the minus ends of m icrotubules in vitro, its ability to translocate purified vesicles on microtubules depends on the presence of an accessory complex known as dynactin. We have cloned and characterized a novel gene, NIP100, which encodes the yeast homologue of the vertebrate dynactin complex protei n p150(glued). Like strains lacking the cytoplasmic dynein heavy chain Dyn1p or the centractin homologue Act5p, nip100 Delta strains are via ble but undergo a significant number of failed mitoses in which the mi totic spindle does not properly partition into the daughter cell. Anal ysis of spindle dynamics by time-lapse digital microscopy indicates th at the precise role of Nip100p during anaphase is to promote the trans location of the partially elongated mitotic spindle through the bud ne ck. Consistent with the presence of a true dynactin complex in yeast, Nip100p exists in a stable complex with Act5p as well as Jnm1p, anothe r protein required for proper spindle partitioning during anaphase. Mo reover, genetic depletion experiments indicate that the binding of Nip 100p to Act5p is dependent on the presence of Jnm1p. Finally, we find that a fusion of Nip100p to the green fluorescent protein localizes to the spindle poles throughout the cell cycle. Taken together, these re sults suggest that the yeast dynactin complex and cytoplasmic dynein t ogether define a physiological pathway that is responsible for spindle translocation late in anaphase.