Bovine seminal plasma proteins and their relatives: A new expanding superfamily in mammals.

BSP proteins represent three major proteins of bovine seminal plasma: BSP-A1/-A2, -A3 and -30 kDa. The BSP protein signature is characterized by two tandemly repeated fibronectin type 2 (Fn2) domains. Although classical affinity chromatography and protein sequencing have proven that the BSP protein homologs may be ubiquitous in mammals and functionally related to sperm capacitation, only the three bovine genes have been reported thus far. In this study, we report three new BSP protein-related genes from bovine, as well as other BSP protein-related DNA sequences from human, chimpanzee, mouse, rat, dog, horse and rabbit. Analysis of the relationships between all Fn2 domain-containing proteins revealed that the Fn2 domains found in BSP-related proteins have special features that distinguish them from non-BSP-related proteins. These features can be used to identify new BSP protein-related sequences. Further molecular evolutionary analysis of the BSP protein lineage revealed that all BSP proteins and their related sequences can be grouped into three subfamilies: BSPH4, BSPH5 and BSPH6, which indicates that the BSP protein family is much bigger than previously envisioned. More interestingly, the three BSP proteins in bovine within the BSPH4-subfamily were shown to evolve rapidly. The ratio of nonsynonymous to synonymous substitutions was higher than 1. The analysis also indicated that the rate of evolution was heterogeneous between the first and second Fn2 domains of the genes. These data may reflect that some amino acids in BSP proteins are under a strong positive selection after gene duplication and that each BSP protein evolves rapidly, possibly to acquire new functions.