A 7-A projection map of frozen, hydrated acrosomal bundle from Limulus sperm

The acrosomal bundle from Limulus sperm has been imaged to 7 A in a 400-kV electron cryomicroscope using the spot-scan technique. These images have been processed by Fourier averaging, corrected for the contrast transfer function, and merged to that resolution. The reconstructed density map of this projection shows features consistent with the 13-A projection map from a helical reconstruction and with the X-ray-derived model for F-actin, and regions of possible scruin-scruin contacts.