1.35 and 2.07 angstrom resolution structures of the red abalone sperm lysin monomer and dimer reveal features involved in receptor binding

Abalone sperm use lysin to make a hole in the egg's protective vitelline en velope (VE). When released from sperm, lysin first binds to the VE receptor for lysin (VERL) then dissolves the VE by a non-enzymatic mechanism. The s tructures of the monomeric and dimeric forms of Haliotis rufescens (red aba lone) lysin, previously solved at 1.90 and 2.75 Angstrom, respectively, hav e now been refined to 1.35 and 2.07 Angstrom, respectively. The monomeric f orm of lysin was refined using previously obtained crystallization conditio ns, while the dimer was solved in a new crystal form with four molecules (t wo dimers) per asymmetric unit. These high-resolution structures reveal alt ernate residue conformations, enabling a thorough analysis of the conserved residues contributing to the amphipathic nature of lysin, The availability of five independent high-resolution copies of lysin permits comparisons le ading to insights on the local flexibility of lysin and alternative conform ations of the hypervariable N-terminus, thought to be involved in species-s pecific receptor recognition. The new analysis led to the discovery of the basic nature of a cleft formed upon dimerization and a patch of basic resid ues in the dimer interface. Identification of these features was not possib le at lower resolution. In Light of this new information, a model explainin g the binding of sperm lysin to egg VERL and the subsequent dissolution of the egg VE is proposed.