Crystallographic analysis of acrosomal bundle from Limulus sperm.


The acrosomal process of Limulus sperm contains a bundle of filaments composed of actin and a 102 kDa protein in a 1:1 molar ratio. The structure of the bundle in true discharge was investigated by electron cryomicroscopy, X-ray scattering and crystallographic image analysis. A bundle can be characterized as a quasi-crystal with continuously varying views along the bundle axis. Each segment of the bundle is found to obey the symmetry of space group P1, with a = b = 147 A, c = 762 A, alpha = 90 degrees, beta = 90.6 degrees, gamma = 120 degrees. A unit cell contains a helical repeat of the filament with a selection rule following that of an actin filament. A 24 A projection map based on the h0l view was reconstructed after averaging 5300 unit cells from six electron images. Filaments in this projection are well separated and clearly display a 21 screw symmetry. This screw symmetry results from the helical parameters of the bundle filament and is found to be a non-crystallographic symmetry element present in the unit cell. Our structural analysis has led to the proposal that the assembly of a stable bundle with a defined maximum diameter can be controlled by the crystallographic packing of the twisted filaments.


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@article{Schmid1991CrystallographicAO, title={Crystallographic analysis of acrosomal bundle from Limulus sperm.}, author={Michael F. Schmid and Paul Matsudaira and T W Jeng and Joanita Jakana and E. Towns-Andrews and J . Bordas and Wah Chiu}, journal={Journal of molecular biology}, year={1991}, volume={221 2}, pages={711-25} }