CRYSTAL-STRUCTURE OF ACIDIC SEMINAL FLUID PROTEIN (ASFP) AT 1.9 ANGSTROM RESOLUTION - A BOVINE POLYPEPTIDE OF THE SPERMADHESIN FAMILY

We report the three-dimensional crystal structure of acidic seminal fl uid protein (aSFP), a 12.9 kDa poly-peptide of the spermadhesin family isolated from bovine seminal plasma, solved by the multiple isomorpho us replacement method and refined with data to 1.9 Angstrom resolution with a final R-factor of 17.3%. aSFP is built by a single CUB domain architecture, a 100 to 110 amino-acid-residue extracellular module fou nd in 16 functionally diverse proteins. The structure of aSFP reveals that the CUB domain displays a beta-sandwich topology organised into t wo 5-stranded beta-sheets, each of which contain two parallel and four antiparallel strands. The structure of aSFP is almost identical to th at of porcine spermadhesins PSP-I and PSP-II, which in turn show limit ed structural similarity with jellyroll topologies of certain virus ca psid proteins. Essentially, topologically conserved residues in these proteins are those internal amino acids forming the hydrophobic core o f the CUB and the jellyroll domains, suggesting their importance in ma intaining the integrity of these protein folds, On the other hand, the structure of aSFP shows structural features that are unique to this p rotein and which may provide a structural ground for understanding the distinct biological properties of different members of the spermadhes in protein family. (C) 1997 Academic Press Limited.