title: Structure of the acrosomal bundle.
abstract: In the unactivated Limulus sperm, a 60- micro m-long bundle of actin filaments crosslinked by the protein scruin is bent and twisted into a coil around the base of the nucleus. At fertilization, the bundle uncoils and fully extends in five seconds to support a finger of membrane known as the acrosomal process. This biological spring is powered by stored elastic energy and does not require the action of motor proteins or actin polymerization. In a 9.5-A electron cryomicroscopic structure of the extended bundle, we show that twist, tilt and rotation of actin-scruin subunits deviate widely from a 'standard' F-actin filament. This variability in structural organization allows filaments to pack into a highly ordered and rigid bundle in the extended state and suggests a mechanism for storing and releasing energy between coiled and extended states without disassembly.
authors.affiliation: National Center for Macromolecular Imaging, Verna and Marrs McLean Department of Biochemistry and Molecular Biology, Baylor College of Medicine, Houston, Texas 77030, USA.
authors.forename: Michael F
authors.initials: MF
authors.lastname: Schmid
authors.forename: Michael B
authors.initials: MB
authors.lastname: Sherman
authors.forename: Paul
authors.initials: P
authors.lastname: Matsudaira
authors.forename: Wah
authors.initials: W
authors.lastname: Chiu
mesh_terms: D000177:Acrosome / Q000737:chemistry*; D000199:Actins / Q000737:chemistry*; D000818:Animals; D020285:Cryoelectron Microscopy; D004548:Elasticity; D006737:Horseshoe Crabs / Q000166:cytology*; D008297:Male; D008958:Models, Molecular; D020836:Protein Structure, Quaternary
chemical_list: D000199:Actins; C083765:scruin protein, Horseshoe Crab