An Mr 63-kD sea urchin sperm flagellar membrane protein has been previously implicated as a possible receptor for egg jelly ligand(s) that trigger the sperm acrosome reaction (AR). The cDNA and deduced amino acid sequences of the 63-kD protein are presented. The open reading frame codes for a protein of 470 amino acids which contains a putative signal sequence of 25 residues. Western blots using antibodies to two synthetic peptides confirm the sequence to be that of the 63-kD protein. The mRNA is approximately 2,300 bases in length and the gene appears to be single copy. The protein is released from sperm membrane vesicles by treatment with phosphatidylinositol-specific phospholipase C, showing that it is anchored to the flagellar membrane by glycosylphosphatidyl inositol (GPI). Although we cannot demonstrate involvement of the 63-kD protein in the AR, it is of potential interest because it shares significant similarity with the developmentally expressed proteins crumbs, notch and xotch as well as human uromodulin over a region that includes two separate EGF repeats.
The histone 2A proteins of the sea urchin Strongylocentrotus purpuratus are compared with those of the mouse. While the major H2As in these two organisms do not comigrate on two-dimensional gels, the sea urchin contains a protein that comigrates with the minor histone 2A variant H2A.Z from mammals. H2A.Z is of particular interest because its sequence homology with other H2As is quite low, and it is not phosphorylated as are other H2As. A comparison of the tryptic peptide patterns of several H2As from sea urchin blastulae and mouse L1210 cells show that, while the patterns of the H2A.Zs differ greatly from the patterns of the other H2As, the patterns of the mouse and sea urchin H2A.Zs are very similar. Since the H2A.Zs have only one or two peptides in common with the other H2As, the conservation of their sequence indicates that H2A.Zs have evolved under somewhat different selective pressures from other H2As. Unlike all the other sea urchin H2As whose syntheses either turn on or off during early development, H2A.Z seems to be synthesized continuously throughout this period.U