Structure of sperm activating protein

Area of Science:

• Reproductive Biology
• Immunology
• Biochemistry

Background:

• Serum is a key additive for human sperm preparation in in vitro fertilization (IVF), significantly improving sperm motility compared to other bodily fluids.
• A major sperm-activating macromolecule in serum was previously purified and identified as a complex of immunoglobulin and apolipoprotein A-I.

Purpose of the Study:

• To further characterize the structure and function of the purified sperm activating protein (SPAP).
• To elucidate the specific interactions of SPAP with human spermatozoa and its potential role in reproductive processes.

Main Methods:

• Partial proteolysis and various immunological techniques were employed to analyze the SPAP complex.
• Bacterial expression products (C23 and ZZ-T) that bind IgG were used to probe SPAP structure.
• Fluorescence microscopy with anti-SPAP antibodies visualized SPAP binding to spermatozoa.

Main Results:

• SPAP specifically interacts with antibodies against immunoglobulin G (IgG), particularly IgG4.
• Apolipoprotein A-I is associated with SPAP but not at the primary IgG binding sites.
• SPAP binds to the postacrosomal region of spermatozoa, enhancing motility and suggesting a role in sperm selection within the female reproductive tract.

Conclusions:

• SPAP is proposed to be an IgG4 molecule with an apolipoprotein A-I molecule bound within the Fab arm pocket.
• SPAP's specific binding to spermatozoa and enhancement of motility suggest a role in sperm function and selection.
• Further research into SPAP's biological role may reveal diagnostic and therapeutic applications in reproductive medicine.