3-DIMENSIONAL STRUCTURE OF THE ACROSOMAL FILAMENT OF LIMULUS SPERM BY400 KV ELECTRON CRYOMICROSCOPY

The acrosomal bundle of Limulus sperm was imaged by electron cryomicro scopy, and the three-dimensional structure of a filament computational ly isolated from the bundle was determined by helical image reconstruc tion. The actin model of Holmes was fit into the map, and its interact ions with scruin, the actin-binding and cross-linking protein, were st udied. Scruin binds to two consecutive actins along the helix via subd omains 1 and 3. These interactions involve helix-loop-beta motifs that are present in both actin subdomains (in different monomers) in posit ions available for binding by the same scruin molecule as it wraps aro und the actin. Taking first the structural motif homology and then loo king for sequence pattern similarities, a stretch of 12 out of 20 matc hes in hydrophobicity is found. Scruin itself has been found to have a n internal tandem homology.