Novel sperm-binding proteins of epididymal origin contain four fibronectin type II-modules

Novel fibronectin type II (Fn2)-module proteins were cloned from human and canine epididymal cDNA libraries. cDNA sequences predicted a highly conserved protein family, related but not homologous to ungulate seminal plasma proteins (approximately 50% sequence identity), and the first known examples of proteins with four tandemly arranged Fn2-domains. By Northern blot and in situ hybridization analyses the encoding mRNAs were shown to be abundant products of the epididymal duct epithelium, but not detectable in other tissues. Homologous mRNAs were identified in the epididymides of various mammals, representing members of this novel protein family of epididymal origin. Within the Fn2-module-encoding stretches, species homologues displayed >85% sequence identity, but showed high variability at their predicted N-termini. An antipeptide antiserum in Western blot analyses detected 30-35 kDa immunoreactive protein bands in epididymal tissue, cauda epididymidal fluid, and sperm membrane protein preparations. The tandem arrangement of increasing numbers of Fn2-modules might functionally correspond to the tendency to form oligomers that has been described for lipid-binding proteins.