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SLLP1, A Unique, Intra-acrosomal, Non-bacteriolytic, c Lysozyme-Like Protein of Human Spermatozoa¹

Center for Research in Contraceptive and Reproductive Health,³ Department of Cell Biology, University of Virginia, Charlottesville, Virginia 22908 Academic Computing Health Sciences,⁴ University of Virginia, Charlottesville, Virginia 22908 Department of Pharmacology and Cancer Biology,⁵ Duke University Medical Center, Durham, North Carolina 27710

We report the presence of a unique, non-bacteriolytic, c (chicken or conventional type) lysozyme-like protein, SLLP1, in the acrosome of human sperm. C lysozymes are bacteriolytic and can also bind to N-acetylglucosamines linked by ß-1,4 glycosidic bonds. Most of the invariant residues (17 out of 20), including all the cysteines, were conserved in SLLP1, but the two catalytic residues E35 and D52 of c lysozymes were replaced with T and N, respectively. The full-length cDNA encodes a protein of 215 aa with a predicted protease cleavage site between A87 and K88. The processed form of SLLP1, which showed an exon-intron organization similar to human c lysozyme, was the major isoform in the acrosome of ejaculated sperm. As expected, based on its sequence, the mature protein secreted from yeast showed no bacteriolytic activity. A significant decrease (54%, P 0.001) in the number of sperm bound to zona-free hamster eggs was observed in the presence of antisera to recombinant SLLP1. SLLP1 mRNA (size, 1 kb) appeared to be expressed only in the testis and in the Burkitt lymphoma Raji cell line. The gene SPACA3 encodes SLLP1 and contains five exons at locus 17q11.2. Because of its typical c lysozyme-like sequence, genomic organization, conservation of putative substrate-binding sites even in the absence of catalytic residues, and localization in the acrosomal matrix, we hypothesize that, after acrosome reaction, SLLP1 could be a potential receptor for the egg oligosaccharide residue N-acetylglucosamine, which is present in the extracellular matrix over the egg plasma membrane, within the perivitelline space, pores of zona pellucida, and cumulus layers.

¹ This work was supported in part by grants from Fogarty International Center D43 TW/HD 00654, NIH HD U54 29099, P30 28934, the Andrew W. Mellon Foundation, Schering AG, and National Institute of Justice (2000-IJ-CX-K013; points of view in this document are those of the authors and do not necessarily represent the official position of the U.S. Department of Justice).

² Correspondence: John C. Herr, Department of Cell Biology, University of Virginia Health System, Box 800732, Charlottesville, VA 22908. FAX: 804 982 3912; jch7k{at}virginia.edu