Structure of sperm activating protein.

Serum is used as an additive in the preparation of human spermatozoa for fertilization in vitro, as it is superior to other body fluids in supporting sperm motility. We recently purified the major sperm activating macromolecule present in serum and showed it to be a complex of immunoglobulin and apolipoprotein A-I. This complex, which we named sperm activating protein (SPAP), has now been further characterized using partial proteolysis in combination with different immunological methods. SPAP was shown to interact only with antibodies against immunoglobulin G and more specifically with those against IgG4. The bacterial expression products C23 and ZZ-T (which bind to specific sites on the IgG molecule) bound in similar ways to SPAP as to IgG4 and did not hinder proteolytic cleavage of SPAP, indicating that apolipoprotein A-I is not bound closely to the binding sites of these proteins. Purified F(ab')2 fragment from SPAP was also shown to contain apolipoprotein A-I, and had a higher MW than the corresponding fragment from IgG4. Taken together, the most plausible (and in our view only possible) structure of SPAP shows an apolipoprotein A-I molecule bound in the pocket formed between the Fab arms of an IgG4 molecule. Anti-SPAP antibodies visualized by secondary fluorescein isothiocyanate (FITC)-labelled antibodies were bound to the postacrosomal part of the spermatozoa, indicating that SPAP is specifically bound to this area, and directly interacts with the spermatozoa. Based on these and earlier experiments it is speculated that SPAP acts in the lower part of the female genital tract. The benefit of SPAP should be at its greatest in these regions, and it is also possible that SPAP exerts a selection mechanism, as those spermatozoa affected by SPAP acquire increased motility, which might be important in order to reach the upper part of the female genital tract. Further exploration of the biological role of SPAP may indicate its diagnostic and therapeutic possibilities.