Identification and Characterization of Putative Receptors for Sperm‐Activating Peptide I (SAP‐I) in Spermatozoa of the Sea Urchin Hemicentrotus pulcherrimus

Abstract

We characterized putative receptors specific for sperm‐activating peptide I (SAP‐I: GFDLNGGGVG) in spermatozoa of the sea urchin Hemicentrotus pulcherrimus, using both binding and crosslinking techniques. Analysis of the data obtained from the equilibrium binding of a radioiodinated SAP‐I analogue [GGGY(125I)‐SAP‐I] to H. pulcherrimus spermatozoa showed the presence of two classes of receptors specific for SAP‐I in the spermatozoa. The incubation of intact spermatozoa as well as sperm tails or sperm membranes prepared from H. pulcherrimus spermatozoa with GGGY(125I)‐SAP‐I and a chemical crosslinking reagent, disuccinimidyl suberate, resulted in the radiolabelling of a 71 kDa protein. The protein appears to be associated with a 220 kDa wheat germ agglutinin (WGA)‐binding protein. A cDNA encoding the 71 kDa protein was isolated from a H. pulcherrimus testis cDNA library. The cDNA was 2443 bp long and an open reading frame predicted a protein of 532 amino acids containing a 30‐residue amino‐terminal signal peptide, followed by the same sequence as the N‐terminal sequence of the 71 kDa protein. The amino acid sequence of the matured 71 kDa protein is strikingly similar to the 77 kDa protein of Strongylocentrotus purpuratus (95.5% identical) and also similar to cysteine rich domain of a human macrophage scavenger receptor. Northern blot analysis demonstrated that mRNA of 2.6 kb encoding the 71 kDa protein was expressed only in the testis. Copyright © 1994, Wiley Blackwell. All rights reserved