A Spindle Here, a Spindle There

During cell division, replicated chromosomes align on the mitotic spindle poised to segregate to opposite ends of the cell. To prevent errors during mitosis, a spindle checkpoint monitors proper attachment of chromosomes to the spindle microtubules as well as tension that presumably exists between the chromosomes and the spindle. Indjeian et al. now describe Sgo1, a protein found on kinetochores (the central region of chromosomes that become attached to the mitotic spindle) that also has a microtubule-binding domain. In mutant yeast lacking Sgo1, chromosomes no longer align correctly on the spindle, and cell cycle progression is blocked. Sgo1 is likely to represent part of the cell's tension-sensing machinery when errors in chromosome-spindle interaction occur. Many tumor cells are characterized by increased genomic instability and chromosome segregation defects and may possess extra microtubule-organizing centrosomes and multipolar mitotic spindles. Quintyne et al. now find that cytoplasmic dynein-mediated centrosome clustering can help to prevent the formation of multipolar spindles in cells containing additional centrosomes. The authors suggest that the generation of spindle multipolarity in transformation may require two distinct steps--centrosomal amplification followed by centrosome separation. V. B. Indjeian, B. M. Stern, A. W. Murray, The centromeric protein Sgo1 is required to sense lack of tension on mitotic chromosomes. Science 307 , 130-133 (2005). [Abstract] [Full Text] N. J. Quintyne, J. E. Reing, D. R. Hoffelder, S. M. Gollin, W. S. Saunders, Spindle multipolarity is prevented by centrosomal clustering. Science 307 , 127-129 (2005). [Abstract] [Full Text]