Abstract #38

Smc3, a component of the cohesin complex, forms a cylinder around the mitotic spindle. Leocadia V. Paliulis, Kerry Bloom. Department of Biology, University of North Carolina, Chapel Hill, NC.
   The cohesin complex (Smc1, Smc3, Mcd1/Scc1, and Scc3) is required for sister chromatid cohesion, which is established during S phase and released in anaphase. We have examined the distribution of Smc3-GFP in mitosis. In prometaphase and metaphase, Smc3-GFP fluorescence is concentrated between spindle poles, with dimmer fluorescence filling the rest of the nucleus (2.23±0.43 times brighter between spindle poles than in the rest of the nucleus). Smc3-GFP is organized into two distinct lobes. Each lobe flanks the spindle axis, and a region of reduced fluorescence between each lobe extends the length of the spindle from pole to pole. End-on views of the spindle reveal that the lobes of Smc3-GFP reflect a cross section through the center of a cylinder with an outer diameter (469±47 nm) equivalent to that of spindle microtubules. The decreased concentration of Smc3-GFP between lobes is apparent in the end-on view as a "hollow" central core of diameter 230±31 nm. The cylinder of Smc3-GFP fluorescence is significantly reduced upon anaphase onset. From this and other studies, we propose that centromere-proximal DNA is tightly packed around interpolar spindle microtubules. We believe that cohesin contributes to this structure by promoting intrastrand cohesion within centromere-flanking DNA. The mitotic spindle in yeast is a tightly packed structure with ordered arrays of microtubules and chromatin fibers surrounding eight interpolar microtubules.

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