STRUCTURE OF SPERM-ACTIVATING PROTEIN

Serum is used as an additive in the preparation of human spermatozoa f or fertilization in vitro, as it is superior to other body fluids in s upporting sperm motility. We recently purified the major sperm activat ing macromolecule present in serum and showed it to be a complex of im munoglobulin and apolipoprotein A-I. This complex, which we named sper m activating protein (SPAP), has now been further characterized using partial proteolysis in combination with different immunological method s. SPAP was shown to interact only with antibodies against immunoglobu lin G and more specifically with those against IgG4. The bacterial exp ression 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 n ot 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 form ed between the Fab arms of an IgG4 molecule. Anti-SPAP antibodies visu alized by secondary fluorescein isothiocyanate (FITC)-labelled antibod ies were bound to the postacrosomal part of the spermatozoa, indicatin g that SPAP is specifically bound to this area, and directly interacts with the spermatozoa. Based on these and earlier experiments it is sp eculated 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 i t is also possible that SPAP exerts a selection mechanism, as those sp ermatozoa 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.