Biology

Shruthi Viswanath, Ph.D
University of California San Francisco, USA

The yeast spindle pole body and its human cell counterpart, the centrosome play integral roles in cell division by forming bipolar spindles that segregate chromosomes. Improper functioning of the centrosome is implicated in diseases such as cancer: centrosomal defects occur in most types of cancer cells. Yet despite their importance, very little is known about the molecular architecture of the centrosome and spindle pole body¹. In this study, we use an integrative structure determination approach to characterize the structure of the spindle pole body core.

In integrative structure determination, structural models of macromolecular assemblies are computed by satisfying diverse sources of experimental data². These data can be sparse, noisy, ambiguous and arising from a heterogenous sample. Hence we use a Bayesian modeling approach³ that objectively deals with these uncertainties in the data. We applied our Bayesian approach to model the molecular architecture of the spindle pole body core comprising of five proteins (Spc42, Spc110, Cmd1, Cnm67 and Spc29), using X-ray crystallography, FRET⁴, EM⁵, SAXS and yeast two-hybrid data. An iterative four stage scheme was employed, which consists of gathering input information, setting up system representation and scoring, sampling models consistent with the input information and finally, analysis and validation of the resulting ensemble. Modeling was carried out with the open source Integrative Modeling Platform (IMP) package⁶.

The resulting model of the spindle pole body core has a precision of 1.5 nm and is consistent with the input sources of data. The model makes a novel prediction about the stoichiometry for the protein Spc29, and hypothesizes that half the protein might form a ring in the nuclear envelope. Spc29 is one of the first proteins to assemble into the spindle pole body, and is critical for cell viability. The model sheds further light on the process of assembly of the spindle pole body and function of its constituents.

References
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2. Alber. F. et. al. Nature 450, 695-701.
3. Rieping, W., Habeck, M., and Nilges, M. (2005). Science 309, 303-306.
4. Muller, E.G.D. et. al. (2005). Mol Biol Cell 16, 3341-3352.
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6. Russel, D., et. al. (2012). PLoS Biology 10, e1001244.