The three-dimensional structure of the Limulus acrosomal process: A dynamic actin bundle

Limulus sperm contains a dynamic macromolecular structure that rapidly exte nds a 50 mu m process called the true discharge. The core of this structure is a bundle of ordered filaments composed of a complex of actin, scruin an d calmodulin. We determined its structure by electron crystallographic reco nstruction. The three-dimensional map reveals an actin-scruin helix that is azimuthally modulated by the influence of the interactions of a filament w ith its neighbors. There are a variety of density connections with neighbor ing filaments involving scruin. Scruin commonly contacts one neighbor, but we observe up to three interfilament connections involving both domains of the 28 scruin molecules in the unit cell. Our structure indicates that prom iscuous scruin-scruin contacts are the major determinants of bundle stabili ty in the true discharge. It also suggests that rearrangements would be per mitted, which can facilitate the transition from the coiled to the true dis charge form. (C) 1999 Academic Press.