Cloning and sequencing of SOB3, a human gene coding for a sperm protein homologous to an antimicrobial protein and potentially involved in zona pellucida binding

We have previously characterized an 18-19 kDa cationic protein, SOB3, that was detected in the epididymis and localized within the acrosome and on the neck region of human spermatozoa, We suggested that it is involved in seco ndary sperm binding to the zona pellucida, The present study describes its purification to homogeneity by preparative electrophoresis and non-equilibr ium pH gradient electrophoresis. Degenerate primers deduced from microseque ncing were used to amplify a specific fragment from human epididymal RNA by reverse transcription-polymerase chain reaction (RT-PCR), This 164 bp frag ment was extended by 5 ' and 3 ' -RACE to obtain the 548 bp full length cDN A, The open reading frame encodes a 170 amino acid protein. SOB3 is a singl e copy gene. It is 98% identical to prepro-FALL39 and 100% identical to CAP 18, two human genes which were initially identified by screening a human bo ne marrow lambda gt11 library, and which encode an antimicrobial protein, N orthern blots of human tissues revealed a 1 kb transcript in corpus and cau da epididymis only, while RT-PCR showed presence of the mRNA in the three e pididymal regions and also in round spermatids, The above results suggest t hat SOB3 has two roles in sperm protection and fertilization, depending on its dual origin and final sperm localization.