CRYSTAL-STRUCTURE AND SUBUNIT DYNAMICS OF THE ABALONE SPERM LYSIN DIMER - EGG ENVELOPES DISSOCIATE DIMERS, THE MONOMER IS THE ACTIVE SPECIES

Lysin is a 16-kD acrosomal protein used by abalone spermatozoa to crea te a hole in the egg vitelline envelope (VE) by a nonenzymatic mechani sm. The crystal structure of the lysin monomer is known at 1.9 Angstro m resolution, The surface of the molecule reveals two tracks of basic residues running the length of one surface of the molecule and a patch of solvent-exposed hydrophobic residues on the opposite surface, Here we report that lysin dimerizes via interaction of the hydrophobic pat ches of monomers, Triton X-100 dissociates the dimer. The crystal stru cture of the dimer is described at 2.75 Angstrom resolution. Fluoresce nce energy transfer experiments show that the dimer has an approximate K-D of 1 mu M and that monomers exchange rapidly between dimers, Addi tion of isolated egg VE dissociates dimers, implicating monomers as th e active species in the dissolution reaction, This work represents the first step in the elucidation of the mechanism by which lysin enables abalone spermatozoa to create a hole in the egg envelope during ferti lization.