The complement regulatory proteins CD55 (decay accelerating factor) and CD59 are expressed on the inner acrosomal membrane of human spermatozoa as well as CD46 (membrane cofactor protein).

The complement regulatory proteins CD55 and CD59 are expressed on the plasma membrane of human spermatozoa, whereas CD46 is only on the inner acrosomal membrane (IAM) which becomes surfaced exposed after the acrosome reaction when sperm assume fertilisation-competence. CD55 & CD59, two glycosylphosphatidylinositol (GPI)-anchored proteins, have been detected previously in some studies also in the acrosomal region of chemically fixed spermatozoa but never demonstrated at this site on unfixed spermatozoa. Dual labelling immunofluorescence and confocal microscopy on fresh unfixed spermatozoa, with minimal subsequent time to fixation, has shown CD55 to be markedly expressed on the IAM, more than on the plasma membrane. However, unlike for CD46, CD55 displayed patchy staining over the acrosome, with some variation between individual spermatozoa. All IAM-associated CD55 was localised within GM1-containing lipid rafts. CD59 was expressed also on the IAM, but in a pronounced granular pattern with more variation observed from one spermatozoa to another. Both CD55 & CD59 were released from the IAM by PI-PLC, demonstrating them to be GPI-anchored. Analysis of acrosome-reacted spermatozoal CD55 by Western blotting revealed a novel single 55 kDa protein lacking significant oligosaccharides susceptible to glycosidases. Antibody-induced membrane rafting and release of CD55 & CD59 in vitro may have influenced previous results. Significant coexpression of CD55 & CD46 on the IAM suggests some functional cooperation at this site.