The species-specificity and structure of abalone sperm lysin

Abalone sperm lysin is a 16 kDa protein that creates a hole in the egg vitelline envelope (VE) to allow the sperm to fuse with the egg. Purified lysin exhibits quantitative species-specificity in the dissolution of isolated VE. The molecular basis for this specificity has been studied by sequencing lysin cDNA and by solving the lysin crystal structure. In the deduced amino acid sequences of lysins of seven species of California abalones 50% of the positions are invariant. The most highly variable and strictly species-specific region is the amino-terminal domain of residues 2-12. The crystal structure of lysin reveals a highly α-helical protein with a novel fold. Two tracks of basic amino acids run the length of the molecule. A hydrophobic patch of 11 residues lies on the opposite surface from the basic tracks. The species-specific domain of positions 2-12 extends away from the helical core. Mapping the species-variable positions onto the lysin structure indicates regions which could be involved in species-specific molecular recognition.