High Affinity Binding of PSP94 to Human Immunoglobulin Suggests Its Possible Role in Sperm Function.

Abstract

Prostate secretory protein of 94 amino acids (PSP94) is one of the most abundant proteins from prostatic secretions and a major constituent of human seminal plasma. It has also been referred to as beta-microseminoprotein and protein homologues to this have been reported in other species. The exact role of this protein in prostate pathphysiology or sperm function has not been unequivocally established. We had earlier reported that PSP94 has the ability to bind to human IgG. The aim of the present study is to delineate the molecular interaction of PSP94-IgG and to understand its possible role in sperm function. Ability of human IgG and its sub-domains to bind to PSP94 was monitored by ELISA. Kinetics of this interaction was measured by surface plasmon resonance (SPR) technique. Recently solved X-ray crystal structure of PSP94 (PDB 3IX0) by our group was used to model the structure of PSP94-IgG complex. Binding kinetics of PSP94-IgG interaction revealed high affinity binding of IgG to PSP94. This interaction was found to be through the Fab domains of IgG. In-silico molecular modeling of PSP94-IgG complex showed that amino and carboxyl terminal beta strands of PSP94 to be most plausible region involved in IgG interaction. To understand the significance of PSP94-IgG interaction in sperm function, in-vitro experiments were carried out. The results reveal that PSP94 is able to prevent the binding of IgG to human spermatozoa suggesting that PSP94 present in seminal plasma might have an important role in-vivo, in protecting spermatozoa from immune attach in the female genital tract. Research supported by grant from Department of Biotechnology, Government of India and Indian Council of Medical Research (SDM). (poster)