LYZL6, an acidic, bacteriolytic, human sperm-related protein, plays a role in fertilization

Lysozyme-like proteins (LYZLs) belong to the c-type lysozyme/α-lactalbumin family and are selectively expressed in the mammalian male reproductive tract. Two members, human sperm lysozyme-like protein (SLLP) -1 and mouse LYZL4, have been reported to contribute to fertilization but show no bacteriolytic activity. Here, we focused on the possible contribution of LYZL6 to immunity and fertilization. In humans, LYZL6 was selectively expressed by the testis and epididymis and became concentrated on spermatozoa. Native LYZL6 isolated from sperm extracts exhibited bacteriolytic activity against Micrococcus lysodeikticus. Recombinant LYZL6 (rLYZL6) reached its peak activity at pH 5.6 and 15 mM of Na+, and could inhibit the growth of Gram-positive, but not Gram-negative bacteria. Nevertheless, the bacteriolytic activity of rLYZL6 proved to be much lower than that of human lysozyme under physiological conditions. Immunodetection with a specific antiserum localized the LYZL6 protein on the postacrosomal membrane of mature spermatozoa. Immunoneutralization of LYZL6 significantly decreased the numbers of human spermatozoa fused with zona-free hamster eggs in a dose-dependent manner in vitro. Thus, we report here for the first time that LYZL6, an acidic, bacteriolytic and human sperm-related protein, is likely important for fertilization but not for the innate immunity of the male reproductive tract.