MAJOR PROTEINS OF BOVINE SEMINAL FLUID BIND TO INSULIN-LIKE GROWTH FACTOR-II

Bovine seminal plasma (BSP) contains a family of four closely related proteins previously designated as BSP-A1, BSP-A2, BSP-A3, and BSP-30-k Da (collectively called BSP proteins). They are secreted by seminal ve sicles and they bind to the choline phospholipids composing the sperm plasma membrane upon ejaculation. These proteins contain two homologou s domains that are similar to the type II structure present in the put ative insulinlike growth factor II (IGF-II) binding domain of the IGF- II receptor. Thus, it was of interest to verify the interaction betwee n the IGF-II and these proteins. The interaction between the IGF-II an d BSP proteins was demonstrated by solid phase assay, analytical gel f iltration, affinity cross-linking, and gel overlay binding technique. Ah techniques showed that I-125-IGF-II interacted specifically with BS P proteins and that the binding could be inhibited with an excess of u nlabeled IGF-II. No structural homology has been found between BSP pro teins and any IGF-binding proteins (IGFBP) identified to date, and the refore, they represent a new family of IGFBP. Since BSP cross-reacting proteins are ubiquitous, we propose that the specific binding of thes e proteins to IGF-II could modulate the activity of this growth factor .