Species-specific fertilization in abalone
The marine mollusk abalone is the most well characterized animal model of fertilization. As a broadcast spawning marine invertebrate with external fertilization, both sperm and egg can be easily collected in enormous quantities for biochemical and functional characterization (e.g. more than 1 gram of the major sperm protein lysin can be purified from a single male abalone). Off the North American Pacific coast exist 7 abalone species (whose common names are often based on the color of their shells) with overlapping habitat ranges that creates substantial opportunity for hybridization; Despite hybrids being viable in the laboratory they are rarely observed in the wild, with molecular recognition between egg and sperm being one of the major factors that restricts heterospecific mating. Compared to mammalian counterparts, the abalone sperm proteome is remarkably simple and consists largely of only three proteins: lysin, sp18, and FITZAP. Lysin interacts with ZP-N domains of egg VERL to permeate the vitelline envelope (homologous to the mammalian zona pellucida, ZP). Sp18 is an ancient paralog of lysin with a similar tertiary structure despite low sequence similarity that is highly fusogenic and likely facilitates sperm-egg fusion. FITZAP is a small intrinsically disordered protein that enables packaging of lysin and sp18 inside the sperm acrosome at concentrations nearing one molar. As all three abalone sperm proteins are small and highly soluble, they are well suited for structural analysis by solution NMR. One barrier to abalone hybridization is the rapid co-evolution of sperm lysin and egg VERL which has resulted in weak but species-specific interactions between the gamete recognition molecules. On-going work in the lab is focused on characterizing the interactions between egg ZP-N domains, structural mechanisms of lysin-mediated egg coat dissolution, and the structure-function dynamics of sp18 with the oocyte plasma membrane.