Membrane Fusion Is Induced by a Distinct Peptide Sequence of the Sea Urchin Fertilization Protein Bindin

Abstract

Fertilization in the sea urchin is mediated by the membrane-associated acrosomal protein bindin, which plays a key role in the adhesion and fusion between sperm and egg. We have investigated the structure/function relationship of an 18-amino acid peptide fragment "B18," which represents the minimal membrane binding motif of the protein and resembles a putative fusion peptide. The peptide was found to mimic the behavior of its parent protein bindin with respect to (a) its high affinity for lipid bilayers, (b) the ability to aggregate and fuse vesicles, (c) the binding of Zn 2؉ by a histidine-rich motif, (d) the tendency to self-assemble, and (e), as indicated earlier, the adhesion to cell surface polysaccharides. Fluorescence and light scattering assays were used here to monitor peptide-induced lipid mixing, leakage, and aggregation of large unilamellar sphingomyelin/ cholesterol vesicles. For these activities, B18 requires the presence of Zn 2؉ ions, with which it forms oligomeric complexes and assumes a partially ␣-helical conformation, as observed by circular dichroism. We conclude that aggregation and fusion involves a "transcomplex" between peptides on apposing vesicles that are connected by Zn 2؉ bridges.

Characterization of the Sperm Molecule Bindin in the Sea Urchin Genus Paracentrotus

“…A conserved multibasic motif RMKR or RTKR (Gao et al 1986) after the 5 0 preprobindin region indicated the proteolytic cleavage site, which has been present in all echinoids studied thus far (Zigler and Lessios 2003a). As observed in other species, this molecule contained a first exon, including a hypervariable hotspot, and a second exon, including the conserved 55-amino acid core with 18 invariable amino acids involved in membrane fusion (Ulrich et al 1998(Ulrich et al , 1999) and a fragment from 70 to 82 amino acids long containing various repeated polyglycine motifs 3 0 of the core. These two exons were separated by an intron ranging between 292 and 358 bp located after a conserved valine, 5 0 from the conserved core region (Fig.…”

“…The conserved core, implicated in fusion between bindin and the female egg membrane in vivo, shows no amino acid substitutions, indicating the action of purifying selection in this region (Ulrich et al 1998(Ulrich et al , 1999. Higher variation in the sequence, including insertions of glycinerich repeats, is found outside the core.…”

“…In sea urchins, the protein bindin constitutes the major insoluble component of the sperm acrosomal vesicle (Vacquier and Moy 1977;Ulrich et al 1998). When sperm undergo the acrosomal reaction, bindin is deposited on the tip of the sperm acrosomal process after exocytosis from the vesicle.…”

Evolution of Gamete Recognition Proteins

“…25 SEPTEMBER 1998 VOL 281 SCIENCE www.sciencemag.org phospholipid vesicles (23). Mammalian and amphibian sperm possess two plasma membrane proteins, ␣-and ␤-fertilin, that are thought to mediate sperm-egg fusion.…”

Speciation on the Coasts of the New World: Phylogeography and the Evolution of Bindin in the Sea Urchin Genus Lytechinus

“…It is the major insoluble component of the acrosomal vesicle and has been implicated in sperm-egg attachment (Vacquier and Moy 1977). A portion of the molecule functions as a membrane fusogen, suggesting that it may be involved in fusing sperm and egg membranes (Ulrich et al 1998(Ulrich et al , 1999. This fusogenic activity is concentrated in an 18-residue portion of the 55-residue bindin ''core'' that is highly conserved among all bindins characterized to date (Ulrich et al 1998;Zigler and Lessios 2003a).…”