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-kDa (collectively called BSP proteins). They are secreted by seminal vesicles and they bind to the choline phospholipids composing the sperm plasma membrane upon ejaculation. these proteins contain two homologous domains that are similar to the type II structure present in the putative insulin-like growth factor II (IGF-II) binding domain of the IGF-II receptor. Thus, it was of interest to verify the interaction between the IGF-II and these proteins. The interaction between the IGF-II and BSP proteins was demonstrated by solid phase assay, analytical gel filtration, affinity cross-linking, and gel overlay binding technique. All techniques showed that 125I-IGF-II interacted specifically with BSP proteins and that the binding could be inhibited with an excess of unlabeled IGF-II. No structural homology has been found between BSP proteins and any IGF-binding proteins (IGFBP) identified to date, and therefore, they represent a new family of IGFBP. Since BSP cross-reacting proteins are ubiquitous, we propose that the specific binding of these proteins to IGF-II could modulate the activity of this growth factor.