3-D structure of single filaments from the acrosomal bundle of Limulus sperm by 400 kV cryoelectron microscopy: Scruin binds to homologous actin subdomains
Scruin (102 kDa), an actin binding and bundling protein, is found in a 1:1 complex with actin in the acrosomal process of Limulus sperm. The protein sequence of scruin is unrelated to any known actin binding proteins but reveals a tandem pair of homologous domains (Way and Matsudaira, manuscript submitted). Sequence comparisons reveal homology in each domain with kelch, a gene in Drosophila that is important in nutrient transport into the oocyte and in the maintenance of a cellular structure called the ring canal, and with MIPP, a mouse placental protein of unknown function. Their similarity with scruin suggests a cytoskeletal function and they may form a new family of actin crosslinking proteins. The actin binding domains in this family of proteins have not been identified.To understand the structural basis of actin crosslinking and bundling, we have been studying the acrosomal bundle by cryoelectron microscopy. The actin bundle of the Limulus acrosomal process is structurally unique because it can be considered as a single 3-dimensional crystal in space group PI, with 28 actin subunits and scruin molecules per unit cell.