Yeast weighs in on the elusive spindle matrix: New filaments in the nucleus.

T mitotic spindle is an impressive cellular machine, responsible for distributing each set of the duplicated genome into daughter cells. Microtubules, the major component of the spindle, are constantly recycled by two turnover mechanisms: dynamic instability of microtubule plus ends and treadmilling, or flux of tubulin dimers toward the pole. Thus, the critical function of chromosome segregation is based on an intrinsically metastable structure. A nonmicrotubule ‘‘spindle matrix’’ that provides a scaffold for microtubules and or motors has long been hypothesized. Recent findings have revived the quest for the elusive matrix, and the Saccharomyces cerevisiae gene FIN1 (1) is the latest entry in this small but provocative field. Chromosome alignment and accurate segregation is no small feat. Duplicated chromosomes attach to microtubules from each spindle pole of the mitotic spindle. Attachment is mediated through kinetochores, a specialized protein complex built at the centromere of eukaryotic chromosomes. Alignment along the metaphase plate occurs through a complicated dance that is no less impressive then the mating rituals of many animals. Microtubulebased motors play key roles in chromosome alignment (2). Motor proteins act at sister kinetochores to ‘‘capture’’ microtubules and align the sister chromatids to opposite poles of the spindle. Sister chromatids move in a coordinated manner; poleward movement of one sister is accompanied by the other sister moving away from its pole (3). The mechanism behind this coordinated movement is poorly understood; however, it is thought to be based on a physical linkage between sister kinetochores. When force is exerted on one sister kinetochore, the motility state of the other sister switches to the direction of the pull, or tension. Underlying the complexity of motor protein– microtubule interactions are the spindle microtubules themselves. The microtubules are dynamically growing and shortening, and individual tubulin dimers within the microtubule polymer are constantly migrating toward the pole. This phenomenon, known as treadmilling or flux, reveals the inherent metastable state of the mitotic spindle. Microtubules can also generate a pushing force against chromosome arms, denoted microtubule ‘‘polar winds’’ (4). Motor proteins contribute to the polar winds and to the fidelity of chromosome distribution by pushing chromosomes that initially associate with one pole (mono-orientation) toward the other pole. The budding yeast, a single cell eukaryote, exhibits a ‘‘closed’’ mitosis. The mitotic spindle is nucleated from spindle pole bodies embedded in the nuclear envelope (Fig. 1). The spindle is completely contained within the nucleus, which does not break down throughout the cell cycle. The haploid spindle contains 8 polar microtubules (pole-to-pole) and 32 kinetochore microtubules (pole-tochromosome), one for each chromatid. Although chromosomes are not visible by differential interference contrast microscopy, the development of green fluorescent protein (GFP) markers at specific sites in the chromosome relative to the kinetochore has allowed visualization of novel features of chromosome structure and behavior throughout the cell cycle (5). Before anaphase, sister centromeres are separated toward opposite poles (Fig. 2). This separation is important for the maintenance of bipolar chromosome attachment and the functional processes of properly segregating the duplicated genome (5–8). Similar stretching in centromere chromatin has been observed in human tissue culture cells (9) and diatoms (10). Kinetochore stretching depends upon microtubule binding and is indicative of tension across the separated centromeres. Tension is required to stabilize the kinetochore microtubule attachment in, for example, meiotic grasshopper spermatocytes (11) and regulates the direction of chromosome motility (3, 12, 13). The separated sister centromeres oscillate relative to the spindle pole body, in a dynamic sequence exemplified by poleward (P) and away-from-the-pole (AP) movement of vertebrate kinetochores in tissue cells (3). The distance between sister centromeres is also dynamic, albeit reassociation of separated centromeres is infrequent. Yeast centromeres align in a metaphase conformation and, upon anaphase onset, move to the spindle pole (5). The preanaphase and anaphase motility properties of yeast kinetochores are remarkably comparable to motility properties of vertebrate cell kinetochores. Thus the underlying mechanisms and complexity of chro-